maybe a place like Fry's would sell this stuff. it's pretty unique, you'll likely be getting it online... I bought mine from circuit specialists
they do sell a black epoxy adhesive in stores, it might work, if you can be patient and apply it in layers. a thick layer of it will take forever to cure, and will likely bust your solder joints. as an adhesive, it's designed to shrink a large amount when it cures, to draw the item being glued together. as a potting compound, it'll just tug at all your solder joints.
use wax or HDPE as a molding substance, and you won't have to worry about using a release agent. or you can use regular plastic and something like cooking spray as a release agent. a metal mold could be coated with wax pretty easy (blowtorch and odd chunks of wax)
good luck!