Resources

The Big Three for finding the perfect forums:
http://www.big-boards.com/
http://boardreader.com/
http://boardtracker.com/

Check Google Adwords Pricing:
https://adwords.google.com/select/TrafficEstimatorSandbox

Please note that the best forum to advertise in isn't always the forum in your industry.
A young mothers forum may be a better target to sell your scrapbook services than a scrapbook making forum.
If you are unsure of what forums to pick, let the experts at InForum Advertising pick for you.

Top Forums per Industry (7 sample industries)

Automotive
http://forums.clubrsx.com/
http://forums.vwvortex.com/
http://www.honda-tech.com/
http://forums.corvetteforum.com
http://forums.bimmerforums.com/

Cell Phone
http://www.howardforums.com/
http://www.cellphonehacks.com/
http://www.sprintusers.com/forum/
http://forums.wirelessadvisor.com/
http://cellphoneforums.net/

Dating
http://www.plentyoffish.com/
http://www.nexopia.com
http://forums.facethejury.com
http://forums.offtopic.com/
http://www.buddypic.com/forum/

Domain & Internet Marketing
http://www.namepros.com/
http://www.dnforum.com/
http://warriorforum.com/forum/
http://www.wickedfire.com/
http://talk.iwebtool.com/

Gaming
http://boards.ign.com/
http://forums.d2jsp.org/
http://forum.ebaumsworld.com/
http://www.genmay.com/
http://forums.somethingawful.com

Retail & Coupons
http://www.fatwallet.com/forums
http://forums.slickdeals.net/
http://forums.dealnews.com/
http://forums.dealofday.com
http://www.mycoupons.com/boards/

Webmaster
http://forums.digitalpoint.com/
http://www.sitepoint.com/forums/
http://www.v7n.com/forums/
http://www.webmaster-talk.com/
http://www.ozzu.com/index.php