My suggestion is to contact the leading kettlebell distributors such as U.S. Kettlebells and Fitness Factory (just to mention two that come to mind).
Perhaps you can also contact Minnesota-based Dragon Door Publications Inc. (Dragon Door CEO John Du Cane teamed with Pavel Tsatsouline, a fitness trainer who formerly trained the Soviet Special Forces, and brought kettlebells to market in the U.S.)
That is going to be tough, it is hard to gauge fitness equipment sales because there are so many different manufactures and distributors in the market, both U.S and abroad. Also, KB’s come in so many different forms, from competition type kettles, to GoFit kettles sold with DVD’s. Again, most companies are not going to post or share product sales, as well, most fitness clubs do not even have kettles, maybe a couple around but not anything significant for you to properly count.