You are a brave soul. First I would have had the gun checked out by a smith that knew something about them. Second .38 special would have been the hottest round I would have shot it in, even then waiting for some palm pain.
Plastic pearl grips may be had at Numrich gun parts. gunpartscorp.com. You may have to call them, they don't list specifically for the derringer but they have misc. grips.
otherwise the brass and aluminum replacments are at bob's.