bigjohn3,
You have the right idea. The advantage of casting your own parts is so that you can make many copies from one original or master. The most popular method is resin casting. Basically you make a master, build a box around it, coat it with a rubber mold, and after the mold cures pour liquid resin into it. Of course there are other methods too including metal, plaster, and so on. It isn't a cheap process to make resin and rubber molds. I recently purchased a starter kit from Bare Metal Foil company. It comes with a book and what you need to do quite a few molds and castings. I would also suggest searching around the web and articles from Fine Scale Modeler for more information about techniques. The Bare Metal Foil link is as follows:
http://www.bare-metal.com/
It will run you about $65.00 plus shipping. Good luck!
Dave