try Adword Keyword research tool https://adwords.google.com/o/Targeting/Explorer?ideaRequestType=KEYWORD_IDEAS#search.none enter your keyword / niche sort by average CPC and see keyword that giving high cost per click
In my opinion if you are serious about keyword research tools, you get what you pay for....most of the time. In my opinion if you are NOT serious about keyword research, you are not serious about your business.
if you are serious about getting good" buyer keywords"-- then you should be willing to get a "pay for" system- and the best one out there is-- keyword samurai- without a dought i!!!