In addition to Pixelation, mentioned above, there's PixelJoint, home to numerous professional pixel and low-spec artists for hire. You can post job listings in the forum, and you should be able to discuss and negotiate pay on an individual basis. You can get an idea of the skills of various artists by browsing the gallery.
As for pricing, individual sprites and basic animations will tend to run you $50–$100 apiece, while backgrounds are typically more costly. It really depends on the style you want to go for, but I think high-quality low-spec art is generally more time-consuming (since it is so detail-oriented) and hence costlier than other forms of game art.
Edit: Here's a great article all about the business side of pixel art: when and where to use it, what to expect from it, and how much it costs. All of the art in the article is by Fool, one of the most (if not the very most) highly skilled and respected pixel artists around.