I use photos from a lot of stock photo web sites for my ebook, websites, etc. for myself and my clients.
Here are some free ones:
http://sxc.hu/
http://morguefile.com/
The one I use most is a paid one. iStockPhoto.com. The selection is much better and more professional than the free ones and the photos usually end up costing me $3 to $7 each depending. Well worth paying the extra for the selection.