Yeah, I'm gonna go ahead and promote Smooth-On as well. I just started working with Smooth-Cast 300 and it's fabulous. Haven't tried any of the others yet, but I hear they're good. Delivery time is a little slow (or at least it was for me), but I believe they sell it at Dick Blick, if you live near one. Depending on what you're making, the trial size was enough for me to do 2-3 decent layers on two separate masks.
As far as casting resin goes, I've been using "Castin' Craft" brand clear casting resin. It works pretty well and smaller projects usually dry in a day or two, though they occasionally come out a bit glossy/reflective... But that could also be my fault. XD You can pick some up at Hobby Lobby, Micheals, Dick Blick... Pretty much everywhere I think. $20 for 16 fl. oz.