What you’re probably looking for is a marketplace, like AdOnion or AdBrite. That isn’t to say that a lot of people don’t sell ads directly, but these places and places like them are littered with people selling ads like this in a lot of different niches. Easier to track campaigns this way, too.
Otherwise, you can possibly (maybe) post a link in the Marketplace for a link exchange. Particularly, the “Want to Buy” section might benefit you.
Contacting 50 websites and not getting a response is very odd – either your email didn’t sound legitimate (did you offer a contact phone number?), your offered price was too low, or you’re contacting websites that are no longer being maintained or updated.