I ask "how" rather than "where" because there are undoubtedly lots of artists that are just plain lousy or unreliable. Let's assume both paying and non-paying (mods, free games/open-source) ...
The biggest problem for me as a game programmer (who isn't also an artist) is getting good quality 3D models. I have checked out some of the 3D model market places like The 3D Studio and Top 3D Models ...
If I understand correctly, there are two major kinds of artists involved in game development - 2D and 3D. I think I somewhat understand, that if I want to go 3D, I should learn some 3D modeling ...