The main means for communication, other than radio, were the HAM trader yellow sheets. In more recent times, magazines like the Old Timers Bulletin of the AWA, Electric Radio, and others have carried ads for Japanese radios , both for sale and for trade as well as wanted. Which are always a good place to find radios and radio related items is still a good place to go. You never know what will show up at a flea market, You have to check them out and keep going back, week after week. Take cash as these people never heard of credit cards and won't take checks.
Gun shows are much the same.
In the 30 plus years I have been going to gun shows, I have never seen very much radio equipment. You can find keys, headphones, coils and other miscellaneous items. It is rare for someone to have radios at a gun show. The militaria shows are a better choice and then most people do not know very much about Japanese radios so they are usually cheaper. Some years back this outfit started taking in items for auction.