A newly discovered favorite of mine is http://stocksy.com - As far as I know it's invite only and/or has a pretty strict approval process for photographers that want to sell on their site. From what I've seen, the quality is often higher than your typical iStock. It's probably a little more expensive than iStock, but nowhere near the cost of Getty where you can spend several hundred to thousands of dollars per photo. The biggest downsides I've noticed so far is that their search/refinement filters are not the best, and they don't have the quantity that other larger stock photo sites do.

Here's an example:

http://www.istockphoto.com/search/text/business/filetypes/ph...

vs.

http://www.stocksy.com/search?src=head&text=business