There are lots of crystal manufacturers world wide - and packages have been pretty standard for a lot of years. Not only in RC, but everything else that uses a crystal.
Also, not all of the crystals are overtone or "harmonic" crystals. Some are fundamentals, easier to shift a little, or "pull" to generate FM. Some are series resonant, some are designed to be used in a parallel resonant circuit with external tuning, etc. While you might be able to find the manufacturer (or A manufacturer) to get crystals from, you'll still need the specific characteristics of the crystal that your particular radio circuit was designed around in order to insure compatibility.
For no more crystals than I need, I'll just let the radio supplier sort that out. I used to get small quantity crystals from a place in Oklahoma, I think it was International Crystal or some place like that.
here's a link to just a few.. there's dozens, if not hundreds more out there...
http://www.arrl.org/cgi-bin/tisfind?...L+MANUFACTURER