I can’t seem to find anything when I search too… But then again I don’t think I have found anything in the game, whenever we come across an undead mage they have a link to Kirin Tor or Dalaran
Remember the quest where we have to kill an apprentice of an undead mage and a dusin of other mages, just because he feel a lot of hate
One poster in this thread on mmo-champion writes about the Undercity Mages Order, and I don’t think it sounds wrong, that this is the place they would belong to.
Edit: Forgot to link page https://www.mmo-champion.com/threads/1955677-Undead-mage
I don’t think there’s any Forsaken-specific mage order, no. They could be either part of the Kirin Tor, a member of any of Stormwind’s academies when they were still alive, or simply self-taught/taught by a mentor of literally any race.