Try here:
http://www.dwrplastics.com/home
You want the RTV Silicon moulding rubber - general type.
They sell it in several sizes but for most hobby use, the 1kg kit is enough to last ages.
For the resin, Polyurethane fast-cast low odour.
Again, various sizes but I buy 1kg at a time.
I mould loads of parts for my scratch build 1/8 cars and this stuff is the best I've found.
Once the silicon/catalyst are mixed you have around 10 minutes 'pouring' time so get your master and mould ready first.
A sheet of shiny plastic, double sided tape, and build your mould 'walls' with Lego bricks.
You can then use them again.
D/S tape your master and Lego to the base.
Mix the silicon, pour, stand it on a warm radiator and it's ready in a few hours.
Mix the resin and catalyst roughly 50/50.
Pour into the mould a SMALL amount and roll it around into the nooks and crannies, even poking little corners with a cocktail stick.
Maybe one day, get a vacuum chamber to suck out the air, but not needed for 'hobby' quantities.
Then, top up the mould with the rest of the resin.
It will be hard in 10-20 minutes.
Great fun...
Roy.