I use West System epoxy. You can order it from Aircraft Spruce & Specialty on-line or buy it from a local marine dealership. Get the slow hardener for more working time. One can add toner cartridge powder from an old printer to dye it black.
West Systems Epoxy from Aircraft Spruce
The thing you have to realize is that epoxy once mixed generates heat and if left too long in a mixing cup it will exotherm and cure faster than normal. When trying to pour it into a mold, one must do it in several small batches or you risk generating too much heat which can cause bubbling from too sudden of a cure.
You could also use industrial two part urethanes or silicons.... We mold parts at work with the stuff sometimes and it can be ordered in various durometers. We buy ours locally from a custom mold shop. Might be able to search on-line for these kinds of products.
Another option would be the product used to glue in modern car windshields... It comes in caulk gun type tubes and is black. The only problem is that once it is opened, it tends to cure in the nozzle. If you are careful, you might be able to plug the end and re-use it again for a while. It drys harder than silicon, but softer than epoxy.
Advance Auto Parts: Window-Weld™ Super Fast Urethane - Black (10.5 fl. oz.) by 3M - Part 08609