I'm sure I got it from Jantz, or the like. I haven't even seen it at Caswell, but they may carry it by now.
I can always come up with a truckload of stuff I could use whenever I place an order, so shipping isn't so out of line.
It is pretty expensive as far as compounds go, but on many materials it makes a big difference. When you figure a tube will last for years, and hundreds of scales, it's a drop in the bucket.