I believe the aftermarket companies make repros. I once had a radiator shop manufacture me a core,which you can do if you have no other luck.
[updated:LAST EDITED ON Nov-18-04 AT 03:03 PM (CST)]Try www.uneedapart.com I found a heater core for my 1949 6400. In fact I had more than one response on it. Jim
I have a heater core that I would sell for $75 + Shipping. It comes with a housing that is in good shape and the fan blade. Heater core has been tested and does not leak. Please email for questions or pictures (gregthurm@yahoo.com).