2. Materials

 Material  Mold  Cast Set Toxic Shelf Life Cost Mold / Cast  library Life Mix ratio by weight Mix ratio by Vol Notes
 Alginate X  Water 0 years $ Short 4 1 warm water
 Plaster X X  Water 1 years $ Many years 1.4 1.5
 cold water
 Concrete  X Water 1 years $ Many years   
 Clay X X Water 1 decades $ Many years   heat softens
 Gelatin  X Water 0 years? $ Om nom nom nom.... 0.04  cold water
 Urethane X X  Thermoset   $$ 10 years   
 Epoxy  X Thermoset   $$    
 Silicone RTV X X  Thermoset   $$                 
 Silicone Tin  X Thermoset   $$ 5 years   
 Silicone Pt X X  Thermoset   $$$ 100+ years   
 Latex X X Thermoset   $ short   
 Polyester  X Thermoset 2  $$    
 PolySulfide X  Thermoset 3      
 Soap  X Thermoform 0 years? $    
 Wax X X Thermoform 1 years $$    
 PLA X X Thermoform  years $$    
 EVA X X Thermoform 0 years $    

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: