I`m looking as well! I got a couple of tattoo gun needle "top hat rubbers" that looked about the same but they are too big! I`ve searched RS Componants but no joy there.
I have stripped out some old KR808 cartomisers and there is a little rubber post insulator that looks about right but I`m not buying new cartomisers just to strip for the insulator!

The only thing I can think of is to email Kidney punchers and ask politely if they will mail an envelope to the UK?
I`m going to do that later tonight and see if I can get a few packs.
AGA Series Positive Post Insulators - 10 pack - Kidney Puncher