One of the best ways is to find 'feeder' sites that are in related but not competitive businesses. Search engines can help you there.
Sites of interest to families, college students, wedding dresses and items you don't specialize in come to mind at the moment. Hmmm, even restaurants and hotel sites might be a source.
Also, what you are doing here helps. Visiting forums. Also, search for 'Affiliate Programs' and related terms on the internet. This will give you loads of sites specializing in promoting affiliate programs.