'Polystone' is Polyester Resin with an inert filler.
In the US most well stocked craft stores stock kits of pure resin and hardener or catalyst. Various fillers like fine marble dust can be mixed in various ratios depending on your end use. I have also used bronze powder, and ground walnut shell.
The downside of this material is the shrinkage that will occur, the off gassing that often accompanies the end product for some time and the brittleness in thin cross sections. It is also subject to failure at glued joints and it is difficult to drill or machine.
It was often the material of choice in the 1970s and 1980s for Model railroad kits and inexpensive decorator items.
The material of choice today for most good quality railroad and military models is Urethane resin. These materials have a much more predictable shrink rate, less off gassing, better machining qualities and better resistance to damage from impact.
These materials are now usually available in better hobby shops and even craft stores.
Although reasonable castings can be obtained with simple molds, some equipment is required for good quality castings. Most typically pressure pots, vacuum chambers, or a combination of both are needed for some level of production.
I hope this helps!
Ken Osen
