In the U.S.
http://www.smooth-on.comFor plastic and resin Sorta-clear 37. You can see the master so that you can make cuts for two part molds.
Black liquid plastic Smooth-Cast Onyx Slow. When you have some experience you can switch to fast. The black color helps you see that air is not trapped in the Sorta-Clear mold.
They have a mold material for metal but most of you seem to like plastic and resin. Metal is harder to cast by hand. You need a spin caster and a rubber vulcaniser for a really high quality cast. The metal will chill and not flow into the mold evenly unless the mold is designed very carefully and vented properly.
I have used alumilite. You can buy it at Hobby Lobby and Hobby Town USA. I have a local Smooth-on retailer. They make a lot of movies in Atlanta so there is a need for a local rep.
http://www.alumilite.com/ProdDetail.cfm ... ting%20Kit