68 Baracuda electrical questions

Welcome!
Probably best bet for finding the black plastic plugs is junkyard, or somebody parting out a car, but most would probably want to sell the complete part of the harness attached to that plug.
If you have the plugs, you can replace just the terminals inside. (Packard connectors)
If looking for plugs/ harness, keep an eye on ForBBodiesOnly (FBBO) & FCBO as well, though the wiring might be similar but different.

Edit: thanks AAndrews, I didn’t know those were available.