Your best bets are necrodude or conjurer dungeons.
http://www.uesp.net/wiki/Oblivion:Necromancer_Dungeons
http://www.uesp.net/wiki/Oblivion:Conjurer_Dungeons
I recommend Fort Entius. Not because the chances are greater than any other conjurer dungeon, but because you can easily get to the 'boss chest' in a minute or two after entering the fort. You can even save, then check and reload if you don't get what you want and repeat. Good hunting! :foodndrink: