Since I've been asked several times, here's what I use for casting and moulding:
For the moulds I use this here (freaking expensive stuff, but easy to handle and good results):
Two Component Silicone
and for the actual casting:
Cast Resin - light brown
Unfortunately it doesn't have a brand, so I don't know if you can get the same stuff somewhere else cheaper. But on the other hand, this Silicone and Resin is very easy to work with, and with a little exercise you get very good results.
I hope this helps you all :)