Yep, believe it or not, eBay is probably the best source of the old radios at this point. The local (brick n' mortar) swap meets and other sources just do not have the supply as these items are getting fairly rare.
The OldRadio guy in Germany (sorry, his name escapes me at the moment) is very knowledgeable and has some nice inventory but IMHO his prices are 2-3X what you can find elsewhere if you're patient.
As Z pointed out, though, unless you're doing a "concours-weenie" type of resto, do you really need a period-correct radio?
I have not updated it in years, but you might find some interesting info & pics here:
http://www.nosubstitute.org/911/radio/I sold off most/all of my collection a while ago...racecars don't need radios, and that's my latest addiction so they had to go
Feel free to email me offline if you have any Q's