I've recently come to find out, apparently, there's more than one controller compatible with the A2S; the RC231 and RC-N1.
From what I can discern, one has Occusync 3 and the other doesn't.

If this is correct, WHERE can I purchase the one that does Occusync 3?

Googling for a RC-N1 (the one I believe does Occusync 3), all results when examined, show the actual control is RC231?

With recent firmwares, the RC N1's protocol now depends on the linked aircraft.  O3+ a/c --> O3+ to RC (like the M3), or O3 a/c --> O3 to RC (like the A2S), or O2 a/c --> O2 to RC.