I have a guitar amplifier under repair, needing several pots that are
printed circuit mounted -- 15 mm diameter with 3/8 bushing and toothed
shaft. Are there any good sources of replacement parts for this sort of
thing? Clearly, the exact fit is important. Neither Digi-Key nor Mouser
have these.

Going to the manufacturer for these parts is hopefully a last resort,
as I would expect to pay a lot.