These are a few sites I've used in the past:
Guru
Found some incredibly talented designers on here a few years back. Worked with folk who have designed for Disney, Universal, West Coast Customs to name drop just a few.
Design Outpost
I enjoy looking through the posts here and seeing what folk come up with, Only used it a couple of times myself and that was for charity work designs.
Bobex
Will add others as I remember them!
Justin