or leave some bread in a damp place...
Seriously: The smooth-on site is awesome, buy one of their starter kits and play around. It can get expensive to make big molds, but the starter kit is around $35 and you can make a bunch of cool stuff with it, and more importantly understand the process.
Also, if inexpensive is important, get some plaster of paris and make molds for wax (or, use wax to make molds for plaster). Using flexible silicone has its advantages, but cost isn't one of them. Plaster is cheap and reliable.