Niche Retail
Look for forums based around the topic, and see what their rates are for placing a banner on their site (usually in rotation with other sites). When ‘newbies’ start asking how to do this or that, where to get this and that, its usually the sites policy to recommend only sponsors, and remove any posts from spammers.
I pay $175 a month to one site and it generates enough business to keep me afloat.
Magazines are similar, usually more expensive, but with the added benefit of being around longer. Think of how long some mags sit on coffee tables. After a month on a website, if you dont ante up again, the ad disappears.
Just target your market, and your ads will be more cost effective