Bruce,
I got it from Hobby Lobby. It's "Castin Craft Clear Polyester Resin" with the hardener.
I tried the "Amazing Putty Casting Resin" for one of the levels because HL was out of stock of the Castin Craft, but hated it. It comes on two equal parts so you can't control the hardening time. In fact, there are still some areas that are tacky and I'm not sure when/if they will harden. I suppose eventually it will, but I want to try to sell the build or give it to the museum and don’t know how long I'll have to wait.
Steve