If you want some thing that is somewhat like if not exactly like original, Ag tractor salvage yard is the place to go. Manufacturers did not make these themselves, but rather used venders for complete units. Thiokol did the same thing. I am betting there is a period correct box heater in as was pointed out a combine for instance. Typically tractors have roof mounted heaters. (Your feet get cold) Combines and forage equiptment that is self propelled and with cabs have floor mounted units as you need.....
There are probably new units out there as well. Take your pick...
Regards, Kirk