You can buy perfectly ordinary plaster of paris. This will be ok for casting. If your woried about the ultimate strength you can mix the plaster with straw or paper shreds. Alternatively you need something like fibre glass resin. All this can be bought from an arts and crafts store.
Ask your own Dentist-he/she can get hold of a bucket of Pemstone or something similar from the lab. technicians who make all their dentures/crowns/bridges.