I know Eyewire and Digital Vision are quite a bit cheaper than Photodisc and have some good stuff. I have a few CDs of each laying around.
Check out Getty Images at
http://www.gettyimages.com. Lots of stuff online you can buy.
Also, if you're looking for extremely cheap or free stuff, check out:
http://www.istockphoto.com (cheap)
http://www.sxc.hu (free)