Is there a vendor who sells Golden Carp?
In Mist of Pandaria (in Valley of Four Winds) there is cooking trainger who helps you to boost your cooking skills. He also sells ingredients needed for those "power leveling" cooking products. However, seems he is missing Golden Carp.
Is there any vendor in World of Warcraft who sells these fishs?
Yes there is - in Valley of the Four Winds there is an NPC named Nam Ironpaw, who sells 'Bag of Golden Carp' for 1x Ironpaw Token.
Nam Ironpaw is located in the Tillers settlement at Halfhill Market and these bags contain five Golden Carp each.