craftgrrl | Casting resin molds

I was walking through Hobby Lobby the other day, and saw a Castin'Craft kit for pouring resin jewelry. They had several other Castin'Craft brand products, and also had a mold with different pendant shapes. The mold appeared to be a candy making mold (it looked like it was just normal plastic, like what you make chocolates or wedding mints in).

I have done some light research, and know that I usually molds for this stuff are made of rubber. I also know that you can spray a mold release agent onto your molds so that the resin shape pops out easier. Not wanting to spend a buttload on latex rubber to MAKE molds, can I use just a normal plastic candy mold, or even indent my designs into non-drying modelling clay? I'm not going to be making anything that I want to exactly reproduce many times. Anyone else have any knowledge on the subject?