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

There is a shop in the Chicago area that @krazykuda has mentioned. I can dig that out. But you'll still need a decent core.

As we started to discuss in the "identify" thread, the amp ratings from Chrysler may or may not be the same as the ratings used by an aftermarket company.
Honestly most rebuilders haven't cared enough to pay attention to the combinations (rotor and stator) to keep the ratings correct, and ot get the best performance. That's been true for years.

There are two places that I am aware of that do close to 100% correct restoration. While I don't think that's what you are asking for, the photos are helpful in identifying alternators that haven't been molested, or at least not too badly.
This restored example from Plumcrazy has all the correct bits (externally anyways)
Approximately correct casting dates, presumbaly correct assembly date code.