Short answer:
You need a mold rubber and a support shell to hold the mold together. Then you would need a roto casting plastic to make your parts.
The details:
The mold rubber can either be a silicone or a urethane rubber. Silicone is favored because the casting plastic won't stick to it. Urethane is cheaper, but you would need to spray or brush a mold release agent onto the rubber or the plastic will bond to it.
I'd recomend using a brush-on type rubber so that you won't have to use as much material. Mold rubber afterall is the most expensive component of any project.
Your support shell can be anything from plaster, plaster bandages, fiberglass, or even brush-on urethane plastic. What you choose depends on your budget and what your skill level is comfortable working with.
The casting plastic needs to be a specific type that will allow you to slush it around on the inside to build up a wall to the appropriate thickness. Smooth On's (company name) Smooth Cast 65D or their Onyx fast will work well for this.
Before you put out a lot of money for your project, I highly recommend going to Smooth-On's website and watching some of their tutorial videos and then buying a few sample kits to try out on a much smaller item first. You can get sample kits for $20 that will let you get some practice in before you make any big mistakes.
Have a look around there and then come back here with any questions, and we'll do our best to walk you through it.
Cheers,
