I wouldn't pay...but that's just me you can try co-op ad places like
Adgridwork I've never tried them...I may when i've time to look into it

Digital Point's Co-op Advertising Network I left them because of a comment the mods pmed me after I reported people selling myspace accounts and other items that weren't even released yet....

We'll be fine here even when people sell dodgy stuff.

...just makes you wonder if that's what they allow in their forums, what do they allow in their ads....

A place I just signed up for...not too sure about, it's kinda odd...but we'll see how it goes
Traffic Swarm

if you want to pay...then there's always places like
Google Adwords
Yahoo Publisher Network
Adbrite