i have bought ads directly for my sponsors, and also for my paysites. i have done this mostly with google adwords, search123, a little with sex.com and hunkhunter.

many affiliates also offer banner tracking, so you'll know which sales come from what traffic source.

there are also search engines where you can buy an annual listing for around $100. of course, the better your pages are optimized, the higher the ranking - but you can keep improving your pages to raise your listings.