I would stay away from little guys. The best way is to buy a small amount of traffic first, install tracking and track your ROI to see if it workd before you invest big. If you are looking for poup, I would also stay away from CPM and only go for CPC based, as there are so many popup blockers out there.
If you are looking for CPC ads on engines, I would recommend the major ones: Overture, Google, LookSmart, Findwhat.
For popups take a look at Gator as they have a huge user base.
Jah: I'd say stay away from these bulk traffic providers. I got about half of the total visits to my site from them, but I noticed little to no conversion from it. You'd probably be better off spending the money on PPC advertising, I certainly am.
Of course, my story could be the exception rather than the norm. I'd appreciate anyone else with experience chiming in, to see if my story is the exception or the norm.
I purchased some bulk traffic from a small company. Main reason I did it was just to get more hits to my site and hopefully build up some LP..anyways, the traffic was suppose to be US and Canada Business Targeted visitors..I was able to log into stats and view the IPs of sites that had been sent to me, and at first I did some whois lookups, and noticed my traffic was all coming from China. So I emailed them and they got me straighened out with only US and Canada traffic, but my conversions have been awful with them..sad thing is the traffic was going to a site that offered free consultation. All they have to do is fill out a free form..So I think im going to stick with PPC traffic as well