I've been down this road several times, and my recommendation is Folyo (http://www.folyo.me/)

I've tried Dribbble, but half of the people available "for hire" are moonlighting (not ideal and wound up with a lot of false starts) and when I put an ad on Authentic Jobs, I was flooded with extremely unqualified applicants.

I also tried Scoutzie and that was the worst of all of them, because it requires you to comb through a bunch of portfolios, and most weren't very good, and the filters weren't specific enough for skillsets.