There are several options to use also. You can use
Clickzs.com or something like
Turbo Traffic Trader. I use something called
ePower Trader, which was kind of the professional version of Turbo Traffic Trader awhile back. It is now owned by the guy that owns Comus Thumbs. Just remember, that the free scripts will normally skim a percentage of your traffic (as payment), so keep that in mind when choosing.