Where to buy correct alternator for a 1969 340 swinger non a/c car

A rebuild shop should be able to test the output at increasing rpm. However my experience is they rarely do this.
The Chrysler shop manual test is done on the car, with the engine at 1250 rpm, and voltage held at 15 V.
To compare with a bench test, the alternator rpm has to be converted to the engine rpm using the pulley ratio.
Also they need to test at 15 Volts, but that may be asking too much.