Kafa Yak

The only Yaks available atm are:

• 
  
• [item=87789] (3k)
• 
  
• [item=87788] (3k)
• 
  
• [item=84101] (which costs 120 freaking thousand)
• 
  

You can buy any of them from [npc=64518] at Kun-Lai Summit.

As for this one, the Black Riding Yak, and the Modest Expedition Yak, they seem not to be available. Yaks were intended to be Pandaren mounts (instead of the turtles). No idea why they backed up, but looking and how funny my yaks walk, I'm guessing they had some issues with the model.

This mount can be bought from [npc=219032] for [currency=2778] in any Infinite Bazaar during the [event=1525] event.
He sells a total of 41 mounts, of which 30 are unique to [event=1525].

Timerunning Additions:
Total amount needed for the new mounts: 103.400 [currency=2778]
[item=213582] - 2200 [currency=2778]
[item=213576] - 2200 [currency=2778]
[item=213584] - 2200 [currency=2778]
[item=213595] - 2200 [currency=2778]
Jungle Riding Crane - 2200 [currency=2778]
[item=213602] - 2200 [currency=2778]
[item=213603] - 2200 [currency=2778]
[item=213604] - 2200 [currency=2778]
[item=213605] - 2200 [currency=2778]
[item=213606] - 2200 [currency=2778]
[item=213607] - 2200 [currency=2778]
[item=213608] - 2200 [currency=2778]
[item=213609] - 2200 [currency=2778]
[item=213627] - 2200 [currency=2778]
[item=213628] - 2200 [currency=2778]
Black Riding Yak - 2200 [currency=2778]
Modest Expedition Yak - 2200 [currency=2778]
Kafa Yak - 2200 [currency=2778]
[item=213626] - 4400 [currency=2778]
[item=213624] - 4400 [currency=2778]
[item=213625] - 4400 [currency=2778]
[item=213623] - 4400 [currency=2778]
[item=213622] - 4400 [currency=2778]
[item=213621] - 4400 [currency=2778]
[item=218111] - 4400 [currency=2778]
[item=213600] - 6600 [currency=2778]
[item=213601] - 6600 [currency=2778]
[item=213596] - 6600 [currency=2778]
[item=213597] - 6600 [currency=2778]
[item=213598] - 6600 [currency=2778]

Original Mist of Pandaria mounts:
[item=94229] - 18700 [currency=2778], drops from [npc=69769]
[item=87777] - 18700 [currency=2778], drops from [npc=60410]
[item=104269] - 38500 [currency=2778], drops from [npc=73167]
[item=89783] - 38500 [currency=2778], drops from [npc=62346]
[item=94228] - 38500 [currency=2778], drops from Oondasta
[item=95057] - 38500 [currency=2778], drops from [npc=69099]
[item=87771] - 38500 [currency=2778], drops from Sha of Anger
[item=93666] - 38500 [currency=2778], drops from Horridon
[item=95059] - 38500 [currency=2778], drops from [npc=69712]
[item=104253] - 38500 [currency=2778], drops from Mythic Garrosh Hellscream
Reins of the Thundering Ruby Cloud Serpent - 50000 [currency=2778], drops from [npc=64403]

You can get the Reins of the Blonde Riding Yak, Reins of the Grey Riding Yak and Reins of the Grand Expedition Yak from Uncle Bigpocket located (65.4, 61.8) at The Grummle Brazaar, due East of One Keg. It is a shame there are no white yaks for sale that are shown all around him. =( I really like the white. I hope they add this soon to purchase or gain somehow as well.