I buy my latex from here:
http://www.delviesplastics.com/mold_builder.htm
BUT, it's only cheaper for me because they are local and I don't pay shipping. There are other brands out there that are cheaper with shipping. Or, check the phonebook under plastics and call around, someone might have some locally.
And to give you an idea, the first gallon I bought made 4 full size skull molds, and half a dozen shrunken heads, and I still have a bit in the bottom (which is why there's a tipped over can in the one pic)