You can always drive it without the heater core. My 51 leaks so I removed it and just bypassed it with my heater hose. I get enough heat radiating up from the floor to take the chill off any Fall drive. Being East Coasters, most of us will be putting our cars away for winter anyway.
If you're dead set on having a working heater, try Old Car Parts in Portland, OR. Also try Classic Heaters in Farmingdale, NY
516-293-2175,
www.classicheaters.comThey specialize in re-coring heaters, and since they specialize, they might be cheaper than your guy.
You can also try a search on Ebay.
Good luck.