Alumilite is once such casting system. I've worked with it, and it's incredible for picking up small details. It's a really neat product. It isn't cheap, but you can buy it online, and prices do vary. Hobby shops -- big ones -- carry it, but it can be really expensive there. There are tutorials on the Alumilite site.
Other casting materials sources -- these are suppliers I haven't bought from -- are Freeman Supply, Farwest Materials and Smooth-On. There are videos and tutorials on their sites, as well.
Another system for making parts is vacuum forming. Check Make Magazine and Instructables for information and leads on making your own low-cost system.