I'd think ebay would be your only hope, and it'll be expensive if you do find one.
Lots of us have built our own harnesses. You might go that route. If you do, my advice would be to lay your hands on an official Honda shop manual for the wiring diagram because the diagrams in the Haynes and Clymer manuals may not be entirely accurate. I did up a nice color diagram for mine, but it's a '77 CB750K.