Factory wiring harness. Where to buy..

What year/model? I may have something you can work with.
Others will disagree, but the simple fact is that all of the replacement, reproduction harnesses out there are made from wire sourced from China.
In other words, the same folks who make the throw away tools you buy at Harbor Freight are spooling the copper in those wire harnesses.
If your car is a keeper, find good original stuff.