I know of a few guys who have small businesses or cottage industries. You can try these for knobs:
Mark Oppat, Antique Audio,
moppat@flash.net
Parts, tubes, inexpensive caps
Dave Frush,
parts2many@aol.com
Mike Koste, Gobs of Knobs,
michael.koste@zurichna.com
Play Things of Past,
http://oldradioparts.com/
See the How to Order page
Many transformers, speakers and other parts
You will want to send a jpg closeup of the knob, plus measurements (helps to put a ruler by the knob in the pic), AND the type of shaft - flatted, round, slotted, etc. - PLUS the radio make and model.
I had a contact in Europe looking for a Philips knob:
Frag'Jan Zuerst -----Ask Jan First(R)
Dipl. Ing. Jan Philipp Wuesten Elektronik
Preiler Ring 10 ; D- 25774 Lehe, Germany
Phone: +49-4882-6054551 Fax : +49-4882-6054552
<http://www.die-wuestens.de> (Deutsch, English, Francais)
mail to <FJZ@die-wuestens.de>
This was in 2006. I did buy 2 knobs from him at a very reasonable price and he was very helpful.
One thing I would encourage you to do with these small businesses is to give as much info as possible in the first call or email. These guys have to put a lot of labor time into inquiries on top of inventory and parts salvaging. Trading 8 emails to get your info, then finding you aren't going to buy the item, really hurts them. They do desperately want to see radios put back together or they wouldn't be in business, so help them to help you.