Help please, Farkas has been following me forever and i want him to leave.

I need to find him a glenmoril witch head, i had dropped all the heads i had a while back to make room for other loot and i had already completed the quest, i have no idea where i dropped them and all the mobs in the glenmoril cavern are still dead and have no heads... is there any other place where i can find one? or any other way i can get rid of Farkas? its really annoying trying to do Dark Brotherhood with some black eyed nord following you around all the time.