I've seen ads on AR15.com, 1911forum.com, and other sites. Most of them are very specific to those forums.
So? It typically takes no more than an hour to concoct a banner advertisement for a site if the advertiser doesn't already have a collection of generic banners in stock. I've made banners in as little as ten minutes for clients.
If I were you, I'd make a list of top notch advertisers you'd like to have, then start going after them.