SmoothOn has the best quality products for casting and resins.
http://www.smooth-on.com/index.php?cPath=1156
They have products for ever aspect of creating castings including life cast, water and specialty products for taxidermy.
Alumilite has good casting products also but I perfer Smoothon.
If you are going to sculpt anything I recommend Celluclay. It works easy, you can sand and paint or seal it and will last forever.
Its very easy to use.

