This would be a good thread to start on
http://www.apug.org/forums/home.php They specialize in film.
Just a few points:
While most of Kodak's cameras for 620 were less than stellar, the Medalist was world class. It apparently can be converted to 120, at some expense.
As one who has used roll film, 120 at present and 127 in the past, rolling 120 onto a 620 spool and then re-rolling the exposed film onto a 120 spool for processing would be a nasty business. Roll film is attached with tape to the paper backing only at the "beginning" of the roll; when exposed the free end is exposed and you'd have to keep the two curling strips, film and backing paper, untangled until you came to the "beginning" of the roll at which you'd attach the backing paper to the 120 spool and wind it and the film onto the spool. All in the dark, and trying to avoid leaving fingerprints on the emulsion. Cursing and sweating all the time.
If the camera in question is to be a user rather than a shelf queen, conversion to 120 if feasible would be a possibility and far preferable to the spool switcharoo.
There are those who cut down 120 film to "convert" it to 127, but you'd need to have the 127 paper backing for the frame numbers if using the red window, or the start arrows for auto framing cameras. Or create your own backing paper from a template of some sort. If you can find really opaque paper. Again, no fun in the dark. As an owner of a Komaflex S, a 127 SLR and a delightful collectible, I curse Kodak for "inventing" formats and then abandoning them.
Exposing 127 and then winding it onto a 120 spool for the convenience of the processor is a fool's errand. Roll film with its paper backing depends on a proper spool with opaque flanges to avoid edge flashing, at the least, of the film. There are several processors who will handle 127, or do it yourself. BW processing is easy and rewarding.
There are, as someone noted, sources of 127 film, and probably 620, but in the case of 127 at really high cost. My beloved Komaflex S needs expensive repair work, the cost of feeding the little beauty dissuades me from the expenditure.
And just imagine trying to feed a camera taking Kodak's 828 film? Forget about it!