Most of the vendors stock repops. Note that some (most?) are vinyl-covered rather than thin real-leather because vinyl is more 'stretchy' and conforms better to the tight curves needed on top of the knob where the logo badge recesses. Leather-grain vinyl also usually wears better than the very thin leather needed.
A second note- many replacement knobs are aluminum rather than OEM white plastic. Metal resists thread tear-out from obsessive owners trying to 'align' the logo by overtightening the plastic knob rather than popping the logo out and re-inserting with a spot of glue.
Note 3- in the '70s, a PI article was published in which a member tailored replacement leather for his OEM plastic knob. The article included a pattern suitable for copying. I don't remember which issue but it was quite early- possibly when the magazine was being run by founder Fred Matsumoto.