Originally Posted by Fire Me Boy!
Four can be aquired from Dremora Kynreeves at the Shrine of Mehrunes Dagon, east of Morthal. The NPC respawn after 7 days
, establishing the Shrine as perhaps one of the most efficient location to gather Daedra Hearts. A fast way to farm the daedra hearts can be seen here.
Just buy 'em from the guy at the mages college.
Most alchemy shops will have one or two, also.