Well here's a method that is essentially free but takes time:
Grab a few basic keywords that would be relevant to the product you're selling. Plug each of them into Google keyword tool, put the keyword lists you find into a spreadsheet. Then search on each one in turn in the various search engines, check the top ranking sites in the natural
results. If they are non merchant sites then get in touch. You can also check the paid search results too and check for which are affiliates, which are merchants or agencies and then contact those that are affiliates.