I was going to suggest Burman Industries, but Sean already beat me to
the punch. I will, however, add this endorsement:
For 7 years, I worked in creature effects, set construction,
propmakikng, MOLDMAKING, and modelmaking for movies, TV, etc.
Burman is THE #1 source used by ALL of the creature effects and model
shops I worked in. The other companies which come to mind are Davis
Dental, in Van Nuys / North Hollywood, Ca., ( a good source for
alginate) and RLS enterprises in Burbank (If he's still around, it's
one guy in a metal garage-type building. He makes MQ-80, which is the
same thing as Alumilite resin. If I understand correctly, he and the
Alumilite guy developed the formula together, but each retained rights
to use the formula after they parted company)
Anyway, Burman has just about everything you'll need. I suggest
getting their catalog and drooling over it for a while. They have mold
silicone, polyurethane resins, (hard two-part plastic) Polyurethane
elastomers (two part soft plastics) polyfoams (both rigid and soft)
and all the supplies you need to go really insane with your custom Joe
work.
Buzz