I Found Pepe!

This achievement is now live confirmed by this post: http://eu.battle.net/wow/en/blog/19913789/it%E2%80%99s-all-about-pepe-new-in-game-outfits-and-plush-13-10-2015

Locations are as follows:

Pirate Pepe: Spires of Arak - 54.1, 83.6 - Screenshot: http://imgur.com/Fz1QSaC - On a rock in the corner of an Arrakoa ruin in the southwest of the zone.

Knight Pepe: Talador - 51.0, 63.3 - Screenshot: http://imgur.com/i41lE0O - In a tent near some friendly NPCs on the upper area of Auchindoun.

Ninja Pepe: Gorgrond - 47.5, 41.3 - Screenshot: http://imgur.com/mLZ4p4i - In a tent of a decimated Iron Horde orc campsite in the center of the zone.

Viking Pepe: Nagrand - 80.1, 50.4 - Screenshot: http://imgur.com/ekCeP5n - On a box in the outer area of Ring of Trials near two dancing NPCs.

Notes: You cannot use a macro to target him. When you click on the Pepe NPC you get a 1 hour buff that puts that specific Pepe model on your head. You must click on the Pepe NPC to gain credit towards the achievement - simply locating him does not count. In addition, when you click on him you receive an item specific to that Pepe. This item, which will be consumed when used, will allow Pepe to sometimes appear as that model when summoned in the future. This applies to both the summoned 'Toy' Pepe and the clickable Garrison Pepe.

Happy hunting.

Edit: As people have mentioned below, the costumes are per character, not per account, so if you want them on each character you need to get them on each character despite the achievement being account-wide.

Knight Pepe - Talador

• Sir Pepe is a knight, you see, but every warrior needs a rest. You’ll find him half-asleep right now near a pile of amythest.
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• /way Talador 51.02, 63.27 Knight Pepe in tent
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• Sitting in the tent sitting on a chest, not the tent with the Dra chick.
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• Nearby Neutral FP: Exarch's Refuge

Ninja Pepe – Gorgrond

• What killed these iron hordies, you thought you knew—but alas, you were wrong. Turns out it was the stealthy Ninja Pepe all along.
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• /way Gorgrond 47.52, 41.30 Ninja Pepe in tent
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• In the middle tent, sitting on a chair.
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• Nearby Neutral FP: Skysear Ridge

Pirate Pepe – Spires of Arak

• Pirate Pepe flew down south to visit his feathered friends. You’ll find him there relaxing in some arrakoa ru-ins.
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• /way Spires of Arak 54.02, 83.60 Pirate Pepe on rock
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• Sitting on a rock at the base on the inside of the rock wall
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• Nearby Neutral FP: Pinchwhistle Gearworks

Viking Pepe – Nagrand

• This Pepe will be easy for anyone to spot. Viking Pepe always thought his dancing moves were hot.
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• /way Nagrand:Draenor 80.15, 50.36 Viking Pepe by dancers
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• East side of the Ring of Trials, sitting on a crate behind some goblins dancing with a Disco Ball
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• Nearby Neutral FP: Ring of Trials

Notes:

• Pepe cannot be targeted by /target.
• To get credit for the achievement, you must click on him, you will get a cogwheel icon.
• When you click on him, you get a 1 hour buff that will put that Pepe on your head, and you will also get an item in your bag that will "sometimes" dress your Pepe when you summon him.
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• You must learn the item looted from Pepe in order for you to be able to "sometimes" summon him in that hat.
• 
  
• Learning said item will only teach that specific character those hats.

--- Edit: OMG I'm a green post now! <3

Also, I made a nice little "guide" to all things pepe so to speak, on his Wowhead Page.

NB! the following is still valid as of August 1st 2015, patch 6.2.0, pre-draenor flying. Will update as this achivement does become available.
UPDATE 17.09.15
From MMO-Champion:

Muffinus confirmed that I Found Pepe! is still not active with Patch 6.2.2. We may get one other costume with A Frightening Friend next month, as it is a Hallow's End achievement.

https://twitter.com/Muffinus/status/641797413561307136

UPDATE 13.08.15:

Pepe got a Hallow's End costume.
http://media.mmo-champion.com/images/news/2015/august/pepeHalloween.jpg

Which means Pepe costumes will at the least finally be available before or on Hallow's End this year :)

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(ORIGINAL POST)
I asked Jordan Powers, the creator of Pepe and working as an artist at blizzard, where the costumes might be found, he had this to say:

"Hidden in the lines of code, I'm afraid! The costumes will be released at a later date. :) Hopefully I'll get to make more of them!"

So yet again, confirmed; Blizzard are most likely trolling us and it's NOT AVAILABLE right now :)

We might as well give it up and just wait, maybe then Blizzard will finally notice how pissed off some of us are at this achivement and that we stopped caring, then they might release a hint or an official statment about when it will finally be available!

Here's a Link to the Tweet:
https://twitter.com/jord_tron_5000/with_replies

Upvote for closure! :)

Update!: Confirmation from another Blizzard employee;
https://twitter.com/Muffinus/status/625757922824290305

@Fizzl_CTR @WarcraftDevs @nite_moogle Apologies for any confusion, no, it is not yet enabled.

cheers, FizzlCTR

~Skysin/Xavius EU