You might try a radiator repair place, as they used to have supplies of the brass water transfer fin tube in 3 or 6 foot lengths that they cut to length to make up or repair the radiators.The pleated material in between the tubes was copper strip accordion folded. That also came pre made up in a number of standard widths.
Before they went to the plastic top and bottom cans or tanks, they were brass stampings. The heater core ones are of course smaller in all dimensions. Tell they what you want to do and most being gear heads will probably sell you some material.
Your post reminds me when I cut up the kids 73 suburban 3/4 ton to save the heater core and radiator for future material.
