I saw a recent thread on sites to buy 3D models. And I know you can buy photos at iStockphotos. What are some other good sites to buy game artwork? Sprites and other images?
I don't have an answer to your question, but I'd advise you to seek help from an artist. If you can't afford one, consider offering a revenue-sharing deal. Using ready-made sprites or 3D models will probably result in a very uncohesive look (unless handled carefully and only as filler content, for which you'd need an artist anyway). IMO, even simple programmer art is better than generic content that wasn't though with your particular game in mind.
If you need a lot of art assests, it's best to get an artist. Using a mish mash of 3d art is not a good idea due to the contrasting styles.
Depends on the engine, but TurboSquid, Gamebeep, and TorquePowered immediately popped into my head as places where you can get pre-made assets.
