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

Functionally Important:
Correct stamped black alternator pulley.
The chromated machined ones don't seem to line up right and have too much interferance fit making them a PIA to remove.

Rotor that matches stator.
This is very difficult to get perfect because the stators are not easily identifiable unless its an unmolested alternator. The differences are in the number of of windings, size of wire, and maybe color of the insulating material or paint.

That said, the rotor's can be differentiated by measuring resistance or current draw. Any that draw significantly more than the shop manual allows are likley to cause problems wit the VR and the wiring over time.
To measure the resistance, remove the carbon brushes and place a probe on each slip ring.

Some rotors I've measured
NOS late 60s rotor: 3.7 Ohms
Rebuilt '70-71 alternator bought from junkyard 23 years ago: 3.5 ohms
In a very used 1973 'square back': 3.5 ohms
From the '73 FSM. Slip ring to slip ring the rotor resistance should be 3 to 4 ohms at room temperature.

In contrast, the rotor in a 10 year old Carquest 7024 square back measured 1.7 ohms! :(
Yes it may be capable of making more power. But its greatly increasing the load on the ignition and field circuit.
It drew 5.6 amps at 12.5 Volts when field current was tested on the car per FSM.
My experience that is about the maximum we should consider acceptable, and certainly not ideal with standard wiring.
It is higher than even the 1973 shop manual allows for squareback.