I get mine from either autogeek.net or autoality.com. I like to support Autoality because they're the only "local" place where I can go into the store to buy stuff. Basically trying to support a local business that's techincally 100 miles from me. But their prices are comparable to the autogeek giant.
I want to mention that there are different grades of clay too. What you find in the kit is a medium grade. I like to keep an aggressive bar around for really neglected or super contaminated cars. It takes a lot less time to clay a car. However, it risks more marring. Again marring is super micro scratches that you can barely see in only in the correct light. Polishing will eliminate these scratches. All clays do this. In fact, anything you do to the car, wash, dry, etc will do this so don't freak out. I have a bar from Meguire's Professional Series. It comes in a huge chunk so I just bite off a small piece and store the rest.
You hold me higher than you should but I appreciate the complement. I've just done a lot of research, learned from someone that is very good at it, and it's just a hobby of mine. Like all my other interests, I like dispelling myths and other misleading information. So if anyone see's I made an error, feel free to correct me. I do make mistakes too.