I've not bought from many because its not really needed, but I do know of someone who you should probably talk to.
"dhw" is this guy who hangs around a lot of webmaster forums. He has a major network of sites and offers loads of traffic to people for a small fee. He's been known for excellent relivant real visitors which come from his network.
You may want to pm him from here.
http://www.peerhut.com/showthread.php?t=19