One good source is Behance (behance.net). A lot of good game artists have profiles and portfolios on there. You can message them directly or post a job for applicants.

Odesk (odesk.com) also has game artists, although the quality is all over the map. You can post a job description and review the portfolios of the artists that respond to find a good fit. They allow you to do a short "working interview" project, too, where you have the artist spend an hour or two creating a piece or two of your game art so you can see how well they will execute your vision, how they are to work with, how fast they can produce, etc.

I haven't used twitter to find game artists before, but I'd image if you look for the right hash tags, you could send out a request for available game artists and get some responses from available folks.

Good luck.