I would seriously suggest NOT pre paying for traffic. The people who have confidence in their traffic always go on commission based (per signup, revshare etc), the people who want you to spend money before results are usually just scammers.
Of course, I'm sure there is always an exception to the rule.
I've found Google traffic to be okay, although it usually takes a few tries at writing an ad before you have any results.
I dont think there is any one solution for decent targeted traffic. Perhaps having a look at DMOZ or some such directory listing might be an idea?