I've used both elance and guru. You can find good paying clients on both, but you will have to wade through a lot of garbage. I'm amazed at the insanely low budgets posted for some of the projects. I always wonder if they get completed for those prices.
You can also try craigslist. It is pretty unorganized and hard to find things, but there are a lot of postings. Also, I'd say build a good portfolio site and get links to it from as many places as possible. Especially from the stock art sites. People contact me all the time for custom work from them.
Finally, you could try putting together a mailing campaign with a promotional postcard and collecting or buying a list of art directors. I've never had much luck with my mailing campaigns, but it's worth a shot.
Hope that helps. Freelancing isn't an easy thing.