I had a mandolin on which several of the tuning knobs just "crumbled."

I had no luck finding replacement knobs from any of "the usual sources." Replacements for guitar tuner knobs were fairly easily found; but apparently mandolinists are less prone to use their instruments defensively in bar room fights, so there's a more limited market.

Since the mando machines were very conventional and easily available, I ended up just replacing the whole set.

I did find one manufacturer who apparently made a wide variety of knobs appropriate for almost any instrument. These people apparently are the suppliers for all (or many of) the tuning machine makers.

Unfortunately, they appeared to be located in S. Korea, and their web site offered no alternate language options, so the whole bit may have been just an illusion. They did have what appeared to be an "order form" but I was afraid I might end up with a superfluous oriental bride if I tried to send one in.

That was a couple of years ago, and occasionally people do update web sites; but since I couldn't read any of it at the time I didn't save a record of how I ended up there.

John