First find websites which buying a keyword free or pay then secondly find keyword which match your content, about this check your local search engines websites.
I'm assuming you mean finding "buying" keywords were visitors will be more likely to convert. Product names are usually very good, and words such as "buy ___" "___ reviews" "cheap ___" "discount _____" and "____ price comparisons" are good ideas for a starting point. Remember that using a product name in a url can potentially get you into trademark trouble, though.
Using these keywords free,buy etc will give benefit for some time, But one thing for sure these may get us to sandbox or near paneling.Because search engines just think that really spamy words