The latter - it is only randomly found. You boost your chances dramatically (assuming you are level 17+) by clearing conjuror and necromancer dungeons. One of the best is Fort Entius; not because it's chances are greater, but because it is so quick and easy to reach the boss chest on the first level of the fort by killing a mudcrab, rat and one conjuror adept. Save game, run in and check. If no luck, reload and try again. A five minute evolution (or less) for each attempt. And of course, all the dungeons respawn every three game days.
I prefer an approach of just systematically clearing conjurer/necrodude dungeons until you get all four pieces. A large quest in itself.
Links to help:
http://www.uesp.net/wiki/Oblivion:Conjurer_Dungeons
http://www.uesp.net/wiki/Oblivion:Necromancer_Dungeons