The biggest problem I’ve found with free stockart sites is that their search engines are horrible and/or they have a zillion clicks to get to the pictures through ads. You search for something like “computer keyboard” and the search turns up everything from pictures of a blue platypus (yea, Agent P) to photos of barn doors. After alot of searching and trial and error, here are some of the better ones I’ve found:
FREE:
http://pixeldef.com (personal use only – see their legal page)
http://urbandirty.com/gallery/ (GREAT textures for free)
CHEAP:
http://istockphoto.com (of course)
Most of the time it is generally better to pay something for them, even if it’s just a small amount, just to save time and effort. The free ones are out there if you look hard enough, though. Good Luck!