Carl,
You will want to do a bit of research first, as there are at least two basic materials you could use to cast your own rubber-like parts. One is a urethane and the other is a silicone; neither are an genuine vulcanized rubber.
The urethane has the advantage in that the resins are relatively cheap and setup quickly (sometimes too quickly!) But it does tend to tear easy, hence the cheap foot rest rubbers one buys that split after pressing them on over the foot pegs. The silicon (often called Room Temperature Vulcanizing silicon rubber, RTV) is much more durable, but the resins are considerably more expensive than the urethane.
Studying the larger supplier's websites for FAQ and Technical Guides (such as
http://www.smooth-on.com/ ) will get you started on how to tackle your rubber casting project. The main issues you will encounter are bubbles, cure inhibition, and more bubbles!
-Doug
