wheel - 3:19 pm on Nov 12, 2011 (gmt 0)
Go Google your topic. Start calling people who are advertising in adsense, or who are ranking at the top of the serps. Call them. Offer them something.
Be aware that forums suck for advertising. So don't expect people to fall all over themselves to sign up for a banner ad. Be a bit more innovative in what you're prepared to offer them.
For example, offer them their own forum section where they can post offers and stuff. Or offer them a page on your site for stuff like that. If you can make their stuff of value to their visitors, then everybody's happy - your visitors like your forum more, and the advertisers will make more money.