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.
The pink stuff is made by smooth on and its called oomo 25, i believe or 30. I didnt care for it. I liked dragon skin 30 from smooth on. You can degas it but you dont have to. I have used it both ways.
I have tried a couple different methods for molds in the last week. The two I made out of UHMW and cutting board both leaked, despite my best efforts to seal them. I am going to try making a form and mixing some setting-type drywall compound as the mold. I just need to figure out a release agent for it.