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

If by "correct" you mean physically similar, then what you want is a round back alternator with one field brush grounded. Pulleys can be changed, single/double. Parts on these have been changed so much over the years that the only way to really know it's output is to spin it up. If by "correct" you mean also correct part number, then you are in for a hell of a ride- I know.