As Valkea mentioned, the PixelJoint and Pixelation fora are good sources for finding pixel artists. You might also try the GDNet forums.
“Unpaid” posts on these sites tend not to be terribly well received. You’ll need to make a very compelling case to convince anyone to work for free, and you’ll get what you pay for. What many developers don’t know is that pixel art is extremely time- and effort-intensive, and animations and tilesets are especially taxing.
The good pixel artists will ask for their worth—a relatively high hourly rate times many hours of work. In my experience, making all of the pixel art required for a complete game will take at least as long as the programming. At best, you might find an upcoming artist looking to do something pro bono in order to gain exposure.
Programmers seeking artists seem far more common than the reverse. If you are serious, your goal as a developer should be to make a fun, high-quality game, even if it looks crappy. With a demo, you can demonstrate to artists that you’re not just looking for someone to give you free artwork, and that you have the skills to possibly make it worth their while.