There's always some on ebay. For the slide type and the plunger type. I've bought from this guy when I can't get to the jumbles.
https://www.ebay.co.uk/itm/SET-OF-4-PETROL-TAP-CORKS-Enots-Genuine-Cork-BSA-TRIUMPH-Norton-Villiers/323337634504?hash=item4b486cc2c8:g:vt0AAOSwy9ZeCzKD

I'm very stubborn, and all my bikes have their correct cork taps, so I'm quite used to changing them when they leak. The usual saying is "Boil them or oil them" I boil mine in a saucepan for a few minutes and also oil them....Soak them for days if I have time. At first the new corks seem too thick. BUT THEY ARE NOT and will squeeze in and be stiff to operate at first. Ron

email (option): ronpier@talk21.com