Silver Covenant Hippogryph is sold by the Argent Tournament vendor Hiren Loresong <Silver Covenant Quartermaster> in Icecrown for 150 . You must have earned A Silver Confidant to have access to this vendor.
As only Alliance players can become exalted with the Silver Covenant, this is correspondingly an Alliance-only mount. The Horde counterpart is the Sunreaver Dragonhawk mount.
Patches and hotfixes
Patch 3.3.0 (08-Dec-2009): Added ground animations for standing, walking, and running
Patch 3.2.0 (04-Aug-2009): Added