In Australia we have Pinkysil (pink,soft and flexible), Ultrasil (blue and not as soft) and others from Barnes.
In the US have a look at
Smooth-On they also have a lot of how-tos and videos.
Also have a look at ebay for RTV silicone. Don't know the quality of the ebay products.
What plastic will you be casting with? Make sure that the mold material is compatable. There are 2 types of RTV Silicone, Tin cure (condensation cure) and Platinum cure (addition cure). I think that PR needs Platinum cure otherwise the castings won't turn out properly, not sure about that.
TonyO