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

Sure, if you want so-called "remanufactured" garbage with a lifetime warranty that means you get to spend your lifetime exchanging dead parts until you finally give up, and if you're gullible enough to trust that when they say "55 amp" that's what you're getting.
I've had one on my car for over two years with no complaints. The key it seems with OReilly at least, is don't get the NEW unit and don't get the cheapest reman. Get the highest cost reman.