AlphaWolf - the ad Tommy sends out is a good content producer - he's been around for quite some time and is well known throughout the industry as one of the best photographers
I've bought from P Markham - he has good usable stuff for the free sites - the bargain basement stuff is pretty much stuff thats already been submitted a time or two but the regular sets will work
For toon, anime etc Exclusive Content is my choice - she runs a good place
content-club is good as well for overall stuff - and you can get some CD deals if youre looking to combine a few sets
eroticshots Ive bought from and been happy - mostly centerfold, star type stuff - although I tried their site and couldnt get their home page