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: