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It is rare to find a completely free forum which doesn't use some form of advertising. Just search for the words you are interested in and see what sites you can come up with.
And it will be easier to use Adwords if you got the cash.
You can also check out the various text link reseller sites to see if they have any good topical sites. Sometimes, the placement of these links may not be the best for clickthrough, but you may find some that are cheap (since PageRank isn't your main concern), relevant, and well-placed on the page.
Ad networks like Adbrite, etc., may be useful.
Clearly, though, Adwords and Yahoo/Overture can provide highly targeted clicks for you, as long as the price isn't too rich for your budget.