There's one for $40/free shipping right now:
Hmm. That's weird, I knew it as part #9750, so I searched for that term. Did not know to search for #70455, which apparently is the same exact thing: https://www.bricklink.com/v2/catalog/catalogitem.page?P=70455#T=C&C=11.
The receptacles that the various Lego things plug into, in the control box? Those are a little formed contact with the housing of the control box making up the connector shell, from what I remember. You could probably cob something together that'd work, but you can't just go buy that connector, as far as I know.
Oh, didn't know that. I do know that 3mm mini-banana plugs fit perfectly into those holes. So the matching 3mm receptacles would do fine rather than using the Lego part. Speaking of which: I take back what I said earlier about "100% perfect". I meant 100% functional, not 100% in appearance. It could be powered by a Raspberry Pi for all I care. I just want to give people a way to run the vintage software (Lego Logo; BASIC; etc.) on their Apple II or IBM PC and connect it to the period 1980s Lego motors/sensors. Is there a easier/modern way to create the middleware hardware?