Again the key idea here is testing - some sites will outperform others by up to 2 to 1. Where do you buy - where do we start. You want sites where your target customers visit. Where to find ezines. There are about 25,000 Newsgroups alive and active, they're also free.
Some groups take explicit advertising some discourage it.
Search for the groups that are relevant to your target audience. When posting ads leave a space at the beginning of your subject line, that way your ad will come up at the top of most peoples newsreaders. If the site doesn't take explicit advertising then contribute. We haven't got much gold from free classified ads. A good way to reach youth markets.