When I decided to start making my own tires, I asked my supplier (Reynolds)
what to use for molds. The answer was the same stuff you use to make the tire, just use a release agent before pouring material into the mold.
I've been using Vytaflex, either 30 or 40 for the back tires, and 60 for front tires.
The 40 works great for molds. 60 is too stiff to get the tires back out.
And don't forget to order some black coloring as well.