Its going back to the states, but a company called West Marine who are on the web sell them, i had a couple of sets of plastic ones a few years ago from them. They did mail them aswell back then. Might be different now though.
Actually Im talking about flare pistols, not starter guns are we on to the same thing?
Over hear there is the old Very Pistol which takes cartridges and I think can still be bought new with a fire arms license - possibly check with Woods of Crediton - they sell alot of navy surplus and may have some used ones. Remember about the license though!