I deal with a number of vintage devices, refurbishing and reselling
them. One of the annoyances I have is dealing with missing or broken
buttons (and knobs, dials, etc.). As silly as it may seem, something
like a missing button cap, that has no functional effect on the device,
can dramatically lower the value and salability of a device.

Now, if there's an electronic component that's broken, I at least know
how to look for it, by its electrical specs or even its model number.
However, if I need to replace a button, I'm lost. Buttons rarely have
useful identifying markings on them. And there must be thousands of
buttons on the market, with many variations on shape, size, physical
characteristics, ..., from dozens, maybe even hundreds of manufacturers.
At least, so it seems from the searches I've attempted. I've been
unsuccessful at finding replacement buttons and such, except in the
cases where I had the luxury of the specific manufacturer and part
number.

How do you find a replacement button? Are there any tricks to it?
Any hints appreciated.