I've used Dreamstime to buy most of the stock photos for my covers. They're probably my top go to site when I'm looking for stock because they have a good selection and their prices are very reasonable. As far as I know though, they don't design covers, just offer the photos.
I go to Deviant Art a lot, mostly for inspirational art (have to be careful looking there though. Lots of stuff you wouldn't want to see.
). I have seen quite a bit of book covers where an author has commissioned an artist. You could probably find someone in a decent price range. I've never looked seriously. There are so many artists on there, I wouldn't even know where to begin.
