This item is no longer available in game.
It was a necessary part of the quest chain for Dungeon Set 2 a.k.a. Tier 0.5. As of Cataclysm, it became useless due to the removal of those quests. It was not until some time in Mists of Pandaria that Blizzard finally removed it from the vendors. (I checked the Horde vendors on the character with Exalted with Argent Dawn. Neither of them offered it for sale.)