The Cenarion War Hippogryph is sold by Fedryen Swiftspear, the Cenarion Expedition quartermaster, at the Cenarion Refuge in Zangarmarsh for 2000g (normal list price). However you must be exalted reputation with the Cenarion Expedition, and by then it will be at a discounted price, of 1400g.
Two hippogryphs can be purchased from the Argent Tournament, each with different requirements.
The high elves offer a Silver Covenant Hippogryph through their quartermaster for 150 to anyone who is exalted with The Silver Covenant (Alliance only. Horde characters who are Exalted with the Sunreavers are offered the opportunity to buy the Sunreaver Dragonhawk for 150 ).
Trading Card Game
The Blazing Hippogryph can be obtained through the Wrathgate TCG expansion set.
The Corrupted Hippogryph can be obtained through the Crown of the Heavens TCG expansion set.
Recruit a Friend
Added in Patch 5.4.1, the Emerald Hippogryph is one of 4 mounts available through the new Recruit a Friend program (there are also 4 pets to choose if you do not want, or already own the mounts).
The upgraded Recruit A Friend allows you to email friends and family members Starter Edition activation keys for World of Warcraft from directly within the game. To get started, simply open up your in-game Friends List and click the new Recruit A Friend button. Similar to the old system, after a recruited friend purchases 1 month of World of Warcraft game time, the veteran player (aka the recruiter) will get 1 month of free World of Warcraft game time. In addition, approximately one week after a recruit successfully purchases two months of game time, the veteran player will be able to select one of the following epic pet or mount rewards to permanently add to their collection.