Andy, most art and craft supply stores will stock Casting Resin and Hardener, also some model shops. moulds can be made either from polyurethane, from the same sources, or, for some small parts, from 'Plasticine' (kiddies play clay).
Basically, you'll need a 'master' from which to make the mould, and this can be, for example, a kit part, such as a wheel.
It's a long time since I made a 'full' mould with the polyurethane, and then only for relatively large parts, as the mould will need to be cut in half to allow extraction of the finished part, or made in such a way it can be 'peeled' back, like casting plater of Paris ornaments.
When using Plasticine for makine, say, a wheel, the 'master' can be pressed into the clay, and very carefully removed, and then repeated for the reverse side. (casting in two halves).
Resin is poured in, with the hardener added per the instructions, and allowed to set. Air bubbles can be a problem but, on small parts, if poured carefully and slowly, after thorough mixing, it shouldn't be a major problem.
Overall though, the materials are fairly expensive, even though the cost has decreased over recent years, so depending what it is you wish to mould, it's often cheaper to buy ready-made parts if available.
Hope this helps a bit.