New Evaporator for ’68 New Yorker AC?

As you may or may not know, I'm in the process of getting a '68 New Yorker back on the road after being abandoned for 40 years! I am going to be removing the dash soon, and I know that THIS is the time to tackle the heater core. I've read some threads here on FCBO about replacing the evaporator. What is it, and where could I possibly find one new? Don't know if it matters, but my car is not an Auto Temp car (thankfully.) I appreciate any help I can get!