The item is currently found as a loot drop from four Rares: Vengeance, Terrorfist, Doomroller and Deathtalon. The drop rate is around 25-30% across the board, but the item can only be obtained with the first kill for each day.
This has likely been implemented to prevent grinding, as Blizzard probably realize the value of the medallion and don’t want to see it flooding their market too fast.
The limited nature of the medallion has made it even more valuable, on the other hand, as players who need the item can’t reliably grind for it forever, they need to go to the Auction House to obtain it. And because of that, those who’re looking to sell off their own medallions can score some nice profits right now.
It’s not clear how long this is going to last, and the medallion is probably going to end up devalued at some point. But it’s still a nice venue for some extra cash right now. Part of being successful in this game is about reading the metagame and knowing when certain items are more valuable in the market. It takes some time and effort, but any serious “WoW” player knows that this applies to the game as a whole, as it’s no casual experience.