If you can't find one (and I just Googled it and I can't) there is an easy way to make one. Go the The Bay and buy a small quantity of polymorphic plastic, there are several brands. When added to very hot water this stuff turns into a thick, puttylike. easily moulded material which will model like Play Dough and sets when cool to a hard polythene-like plastic. It's easy to work when warm but doesn't sand well so it's best to surface heat it and scrape away layers.