Tin-cured or platinum-cured silicone molds are best to use when casting urethane foam, since no release agent is required in a silicone mold. You can also apply a light coat of primer paint to the inside of the silicone mold when casting rigid foam parts. Your cast part will come out of the mold with a coat of primer paint, making it much easier to apply paint.
Polyurethane rubbers can also be used to cast foams, but they will require the use of PolyCoat Sealer and Release Agent, or a Paste Wax. Foam can also be cast in steel, aluminum, epoxy, plastic, and even wood molds, but a paste wax must be used to prevent sticking.
Also, consider the fact that casting foam generates a lot of internal pressure, so the mold must be designed to withstand this pressure.