http://www.dreamstime.com/
http://www.bigstockphoto.com/
http://www.fotolia.com/
http://www.canstockphoto.com/
http://www.stockxpert.com/
http://www.123rf.com/
http://www.istockphoto.com/
 
If you have a little more money, I like Getty Images:
 
http://www.gettyimages.com