And voices in the sky shall cause us all to die

 Another guide no one asked for.

So there are a few really useful accessories in Final Fantasy X-2 that are kind of a pain to get, mainly due to conflicting stories in various places on the internet about how exactly you get these things. So I'm here to help with that.

Ragnarok
Why you want it: Gives the Spellspring auto-ability, making the cost of all spells become 0 MP. So feel free to spam Flare and Ultima as much as you like with this on.
How to get it: You need to implicate Rikku in the Mi'ihen Highroad mystery. Here's a checklist of how to do just that.
 - In Chapter 2, go to Mi'ihen Highroad and enter the Travel Agency to find a familiar face asking about chocobos. Exit and go to the cliff edge. Talk to Calli to begin the Cuckoo for Chocobos mission. During this mission, make sure you do not capture the chocobo during the "blocking" section (when you have to predict which way it will run). Just mess up a few times to ensure that the mission continues. Continue with the mission and at one point you should see Rikku jump down from a high place and fall on her butt. Make sure you get to the Chocobo Eater before the time is up and you should see another scene of Rikku running along the bridge towards a hover at the end of the mission. You need to see both these scenes in order to make implicating Rikku possible.
 - In Chapter 3, go to Mi'ihen Highroad and do the Machina Mayhem mission. During the mission, Shinra will place a Commsphere outside the Travel Agency. When doing the mission, make sure you battle the machina across the gap in Newroad. You'll see a short scene of the Gullwings jumping across the gap to engage the machina. (The scene can be tricky to trigger - just run around the general area and keep mashing X until it happens.)
 - After the mission, go back to Mi'ihen Highroad and go to the cliff between Newroad and Oldroad to see Shinra place another Commsphere there.
 - In Chapter 4, call Rin on the Travel Agency Commsphere and participate in the Mi'ihen Highroad Mystery mission. During the mission, do the following:
 - Switch to Commsphere 3 and wait until a machina appears. Call Rin.
 - Switch to Commsphere 4 and call Rin. He should notice skid marks leading into the ravine below.
 - Switch to Commsphere 7 and call Rin. He should notice footprints on the ledge.
 - Switch to Commsphere 5 where you should see three machina helping an injured hover driver. Call Rin.
 - Switch to Commsphere 1 to see Rin talking to the hover driver. He will automatically talk to you after a few seconds.
 - Switch to Commsphere 2 to see someone repairing the Travel Agency roof. Call Rin.
 - If all the above is done correctly, then during the Culprit Found? mission in Chapter 5, Rin should accuse Rikku, which will reward you with the Ragnarok.

Accessory: Soul of Thamasa
Why you want it: Gives the Magic Booster auto-ability, which boosts the power of magic in exchange for increasing the MP cost. Pairs well with Ragnarok.
How to get it: Complete the Operation: Monkey! mission in Zanarkand in chapter 2 or 3. A treasure chest will appear in Zanarkand Dome containing this accessory. The mission is pretty easy to complete - just look for monkeys who have names that go together. There's no penalty for talking to the wrong monkey, other than they might steal a tiny amount of your gil.
You can also get one of these by buying it from Argent, Inc. for 12000 credits if your publicity level is 4 or higher.

Accessory: Key to Success
Why you want it: Gives the Double All ability, which does exactly what it says - it doubles everything. Item effectiveness, AP, EXP and Gil earned. Very useful.
How to obtain: Here we go.
 - In Chapter 1, complete the Shave the Hypello? mission at Moonflow. Make sure you initiate the mission by speaking to Tobli near where you beam down from the airship. During the mission, make sure you do not let a single bandit escape.
 - In Chapter 2, complete YRP, the Scalpers Three! by selling all ten tickets. It doesn't matter if you make a profit, you just have to sell every single ticket. Here's how to make the biggest profit.
 - If you spoke to Tobli first before completing Shave the Hypello?, Tobli will cut the base price of the tickets in half, allowing you to make even bigger profits.
 - Sell to the woman at the southern exit of South Bank Road for 1500 gil.
 - Sell to the woman with a bandanna at South Bank for 1500 gil.
 - Sell to the woman next to the Save Sphere at South Wharf for 1500 gil.
 - Sell to the man sitting on the stairs at Southern Dock for 1500 gil.
 - Sell to the man in the southwest corner of the same area for 2000 gil.
 - Sell to the girl in the northeast corner at Northern Dock for 2000 gil.
 - Sell to the boy next to the Hypello in the same area for 2000 gil.
 - Sell to the woman at North Wharf for 2000 gil.
 - Sell to the first man you see at North Bank Road for 1500 gil.
 - Finally, sell to the man talking to the old lady outside Guadosalam for 2000 gil.
 - Also in Chapter 2, complete the mission in Macalania Woods.
 - During Chapter 3, do NOT defeat Garik at Mt. Gagazet. Just skip Mt. Gagazet altogether.
 - In Chapter 5, go to the Moonflow and choose to go up on stage. View the scene from all angles, then exit and you should get Episode Complete! If you get Episode Concluded, then you did something wrong along the way. Once you get Episode Complete, go to Tobli Productions in Guadosalam and you'll find Key to Success in a chest there.

Accessory: Invincible
Why you want it: Gives the Break Damage Limit auto-ability, allowing the damage cap to reach 99999.
How to get it: In chapters 1, 3 and 5, talk to the cameraman filming the new Youth League base in Kilika and say yes when offered to look at it. In Chapter 5, he'll take you there by boat. A treasure chest at the base contains the Invincible.

Accessory: Iron Duke
Why you want it: Raises all stats by 100 (except Agility and Luck, which get raised by 10 and 50 respectively), doubles max HP and MP.
How to get it: Well... that's the bad news. This accessory is dropped by Trema, the superboss that awaits on the 100th floor of Via Infinito. So... uh... good luck. There are many strategies for beating Trema, so just pick your favourite one and go for it. In the original version, you can cheese him with the Cat Nip + Trigger Happy strategy, but in the HD version you'll have to be a bit more clever about it.

Accessory: Cat Nip
Why you want it: Gives the SOS ???? auto-ability, causing all attacks to do 9999 damage when the user is on low HP. In the International and HD versions of the game, it also inflicts Auto-Berserk, limiting its usefulness, but can still be useful if given to a Thief with First Strike (either learned naturally or obtained from the Highroad Winds Garment Grid, which is obtained from completing the Clean Sweep mission in Mi'ihen Highroad in Chapter 3).
How to get it: Defeat Black Elemental on floor 40 of Via Infinito.

Accessory: Force of Nature
Why you want it: Gives Omnistrike, which adds all elemental affinities to your attacks, with the ones enemies are weak to taking priority. So you're basically guaranteed to hit a weakness and get extra damage with this. It also absorbs all elements.
How to get it: Complete New Cave in Thunder Plains three times. Keep a piece of paper or Word document handy or something, as well as a calculator. A guide is also recommended.

Accessory: Speed Bracer
Why you want it: Auto-Haste. Enough said.
How to get it: Various ways. You get one for defeating Black Elemental and one for completing the marriage quest at Calm Lands. They're also dropped by Oversouled Lacerta, which can be found on floors 81-99 of Via Infinito. You can also Bribe them from Chocobos in the Fiend Arena if playing the HD or International version, or obtain one by completing the Youth League Tournament.

Accessory: Tetra Bracelet
Why you want it: Gives Tetra Eater, which absorbs fire, ice, lightning and water attacks. Not as good as Omnistrike, but still useful and much easier to get.
How to get it: You need to get Episode Complete in Guadosalam, so here we go.
 - Chapter 1: Go to Macalania Woods and go to the place where you fought Spherimorph in the first game. You'll find Tromell here. Speak to him four times.
 - Chapter 1: Locate the three musicians (Baralai, Donga and Pukutak) in Macalania Woods and speak to them.
 - Chapter 2: Gather three Leblanc Syndicate uniforms and complete the Leblanc HQ infiltration mission.
 - Chapter 3: Defeat Garik at Mt. Gagazet.
 - Chapter 5: Find Tromell in Guadosalam and speak with him twice.
 - Once this is all done, you should get Episode Complete. After that, check the locked house in the northwest corner to find it open. Inside is a chest with Tetra Bracelet.
Note: You can also get these rarely by Bribing Oversouled Zalamander, or by Bribing Critical Bug. You can also get one from the tower calibration minigame in Thunder Plains, or in the HD/International version from winning Youth League Tournament at the Fiend Arena.

Accessory: AP Egg
Why you want it: Gives Triple AP, which does exactly what you'd expect. Useful for making out those Dresspheres!
How to get it: It's found in the Ruin Depths, which... well... okay. This place is a pain to unlock. It requires doing some very specific things with chocobos. ...No, not like that, you pervs.
 - First off, in Chapter 1, complete the mission at Mushroom Rock Road, then talk to Clasko and allow him to board the airship.
 - When Clasko runs off to the Calm Lands, follow him to the old Monster Arena and complete the mission there.
 - You can now catch chocobos. Catch four level 1 chocobos and make sure they all have Bold nature and all have a max level of 5. You can check this by talking to them in the stables. Best thing to do is catch four chocobos in Kilika, Thunder Plains, or Bikanel, then go check them. If any are not Bold, release them. Continue until you have four Bold chocobos, and also make sure all of them have a max level of 5.
 - Assign these four chocobos to your Runners. Send them all to Calm Lands.
 - Fight seven random battles.
 - SAVE YOUR GAME. Talk to Clasko and make sure all four chocobos return. If any of them ran away, reload and talk to him again.
 - All four chocobos should have levelled up, so restore their Heart back to full.
 - Repeat the above steps until they're all level 5.
 - Now, send Runners 1, 2 and 3 to Calm Lands. Leave 4 in the stables.
 - Fight seven random battles.
 - Save and talk to Clasko, make sure they all come back safely.
 - Next, send Runners 2, 3 and 4 to Calm Lands, keep 1 behind.
 - Again, fight seven random battles.
 - Save and go back to the ranch and you should see a scene where Clasko tells you a dungeon has been discovered. That dungeon is the Ruin Depths, and it contains the AP Egg and the Amazing Chocobo, though you'll have to fight Anything Eater in order to get it. I recommend using the map here http://db.gamefaqs.com/console/ps2/file/final_fantasy_x_2_chocobo_dungeon.gif to help with navigating the dungeon.

Accessory: Enterprise
Why you want it: Break HP Limit. Kind of situational, but can be very useful.
How to get it: Skip the Moonflow mission in Chapter 1, which will cause Tobli to board the Celsius in Chapter 2. Push him and the other musicians into the elevator. This is a huge pain to do because of how awkward Tobli is to push - my best advice is to kind of wedge him between your character and the side and then slooooowly push him towards the elevator, being wary of the camera angle change. When you finally get him in, you'll get an Enterprise.