I would recommend AdWords as well. I assume you are most interested in keywords that people are searching for on the Google network, which is what the AdWords tool gives you. Overture gives you data from Yahoo, which is probably relative to Google, but who knows. I’ve never fully understood how WordTracker gets their data, but it is supposed to be more reliable than Overture data.
However, back to AdWords. You will need an AdWords account, so it would be good if you had a site outside of the gambling industry so that you can play around with the tool. You can play with any keyword terms you desire from within any AdWords account. The tool doesn’t give exact numbers, but rather a set of bar graphs that tell you the relative volume of the keyword, the competitiveness of the term (less appropriate for us since affiliate and online gambling advertising isn’t allowed), and one other very nice set of graphs that show you the seasonality of keywords (ie: the word “sportsbooks” is much more popular right now than it is in May).
And I should probably mention the tool is free to use. You can even setup an account for any website for free and never activate it.