Greenstuff is an air drying epoxyclay. It's a little expensive, so you can also use Pioneer epoxyclay. As for the prices, they might vary from site to site, so please feel free to use the internet to check Games Workshop for the items you want. I can then help you contact the owner of Neutral Grounds so you can order. It's a great tool, you know. Also check out local hardware stores for Pioneer aqua epoxy clay. Tamiya also has epoxy putty, but it's out of stock. I don't know how much it costs though.
As for my dragon project, I'm hitting a little snag about sculpting the scales. The wood carving tools I bought are great for carving the hardened putty, but I need finer tools for the small scales. Pics to follow.
Cheers!