2. Materials

Mold / Cast - it is possible to use nearly any material for either, given the correct set of conditions. The list is "common" use. 

NFPA 704 - lists toxicity here under "health" https://en.wikipedia.org/wiki/NFPA_704 from "0" as no risk to "4" as lethal.

Where to get theses materials?

Thermoset platics http://www.reynoldsam.com/product/

Water based http://www.homedepot.com/

ThermoForm Plastic http://www.michaels.com/

All the listed materials have ranges of hardness, Here is a cart to give an idea of where to be: