Let's face it: molding is an expensive hobby. Materials aren't cheap, tools are expensive, and it takes time to make good molds. It's worth it though to see your army of mass produced thing-a-ma-jigs.
But let's say you only need to make one casting. Do you just want to try this hobby?, Do you really want to track down specialty silicone from obscure vendors in your area? Is it even available?
Here's how to make your own molding silicone from what you can find at your local big box store.
You will need the following materials:
100% Silicone caulking
Plastic Cups and Silverware
Mold Release (Also Optional, but highly recommended)
UPDATE: Cornstarch helps molds to try faster.
UPDATE!: Thanks to JDUFFY54, he has provided the following helpful additions to this recipe.
JDUFFY54: I found that using 2 parts cornstarch, 2 parts silicone caulk, and one part mineral spirits works well, more than doubles the ammount of molding stuff you get per caulk tube, and dries compleatley in under an hour. Not to mention, cornstarch is cheaper than caulk, so half the mold is cheaper. I haven't done a lot with it yet, but I have made some test molds which turned out excellent.