I don't think I have ever seen a way to buy traffic other than buying ads on sites that share your subject. Most of the sites selling "traffic" send you tons of junk traffic that get next to know conversion. Mostly bots and exit traffic. Even if real people come to your sites from one of these your visitors will be quite hostle toward you.
Just searching to find sites in your topic area and then looking at the site for banners or what look like paid links would be a good start. Don't be discouraged if you find a site that looks perfect but doesn't appear to be showing ads right now - contact the owner and see if they will run your ad or link, in return for advertising or linkage on your site, or for a payment you can afford.
You can also check out the various text link reseller sites to see if they have any good topical sites. Sometimes, the placement of these links may not be the best for clickthrough, but you may find some that are cheap (since PageRank isn't your main concern), relevant, and well-placed on the page.
Ad networks like Adbrite, etc., may be useful.
Clearly, though, Adwords and Yahoo/Overture can provide highly targeted clicks for you, as long as the price isn't too rich for your budget.