As a producer of gaming miniatures, I have used Smooth-On products and Alumalite. I prefer Smooth-On because I find the products to be of a higher quality and broader range of strengths, rubber types, etc.

Although I am in the business of casting, I am sure that most distributors will sell to modelers. Smooth-On produces kits for the hobbyist, so chat with them. I think a kit should run you between $25 and $40 USD.

I don't have the spec sheet in front of me, but I can get you details if you like. Just PM me.

Cheers!