Try T W Chambers -
Firearm Spares - Shotgun Spares - Shotgun Spares by Make - AYA - T. W. Chambers & Co
But any competent shotgun gunsmith should be able to make one very easily.
Indeed take a piece of silver steel, chuck it in a drill, hold drill in a vice, and set it running. A bit of work with a file and you will soon have a a blank made.
A bit of fitting. Then heat it to a cherry, quench in cooking oil. Polish up. Then temper by heating from the back end, with back end just turning blue and tip straw coloured. Quench again in oil to stop and fix tempering. You might need a few goes, but silver steel rod is not expensive.