You can get Flex Seal in gallon cans, but since it's an infomercial product you'll pay out the butt and from everything I've gathered, they're pretty fast and loose with just how good it is in the ads.
I personally don't think a product like this would be good for the mold proper, but it would be excellent for an outer mold that you didn't necessarily want to be fully rigid but did want some strength from.
I like silicon caulk myself. Once you learn the tricks it becomes a great tool, such as mixing in a drop of acrylic paint to cure it and using naptha to thin it. I've made many molds with it and I've really had good results. Stinks though, that's the only thing.
-Rog