The cover for my novel, which may be the most professional bit about my novel, was done by an artist I found through 99designs. I loved the process, I had over a hundred designs to choose from, tons of variants of these, etc. And I'm super happy with the result!

Run a competition on 99designs.

You should consider 4chan, those guys can be pretty unique in their art choices.

I was lucky enough to get one via contacts. I'd expect though that you can find designers online and see their portfolios before making a serious commitment.