That's Room Temperature Vulcanizing rubber. It's used as head gasket sealant, among other things; try an auto parts store. You should be able to buy it in big caulking gun tubes, if you happen to need that much.
I'm assuming you know how to make a two part mold? Brush on a few layers of liquid latex (to catch the fine details), then use the gasket rubber as filler. You'll have to poke around to remove the air bubbles (the stuff's too thick for them to float to the top). Let it dry for a good long time, then repeat for the other half. Does the trick.