I use iStock, but not happy about it. The good stuff is being added to the premium and vetta collections and it's hard to find something that looks good and isn't at least 10-20 credits.
One thing I've noticed is that if you find a particular image you like but they want too many credits for it - you can google the photographers username and chances are they also have accounts on other istock clones. I did this a while back for a series of images I wanted and ended up saving a good bit because I found the exact same set on an alternative site that were much cheaper.
Alternatively you can use tineye on the comp image to find out if other sites are selling it as well.