I'm new to actually making games, but I work as a professional illustrator, so I know a thing or two about making the art for them. Here are a list of sites that I use when looking for work, so if you want to hire an artist, I'd suggest putting a post up on these sites.
Very huge pool of artists. It's world-wide, so you can get extremely cheap artwork from countries with kick-ass exchange rates if you don't mind the language issues. Clients leave feedback that all future clients can check out, so you can see if the artist sucks before hiring them. Sprite sheets go for cheap on here, mainly because a lot of artists in Malaysia and whatnot have tons of them stocked up. Make sure you specify if you want custom art, because a lot of "artists" on here are really clearing houses that buy up art from starving artists and then resell them.
By far the best site I use. Very professional artists, the ability to make the jobs hourly or commission, and an escrow pay system that makes sure that neither you nor the artist can screw each other over. There are a ton of great international artists on here. Seriously, if you want to hire a great artist, this site has the most and has the best way to ensure everyone goes home happy. I do 80% of business here.
This site is strictly a meeting site. No payment or mediating is done through here, that's between you and the artist. It's hit or miss here, really. As an artist I've been burned by clients and I've also heard of other clients being burned by artists. This is more like a step above DeviantArt but a step below the others.
This one is a lot like Elance, only with more options for how you hire. I think it's a bit too complicated, but check it out anyways. It has a similar set-up to Elance in terms of pay methods and hourly/commission.
Also, if you are good enough at this game-making thing and you think you can do it good enough to get paid, Elance, Freelancer, and oDesk always have postings with clients looking for people to make browser, iOS, and Android games. So while you look for an artist, go ahead and look for work.