Probably better to do it your way, but instead of asking for the harness so you can cut it out of another, mention you're just looking for some Red w/green wire left over. Maybe someone's got a ragged out harness that they don't want to sell/give to someone because it's so ratty, but that bit of wire you're looking for is still in good shape.
Might widen the possibilities for your search.
And in a pinch if you really want it to look stock you can simply buy a short length of Red w/green wire from one of the wire suppliers.
Not every one has stripe options, but there are some online that actually stock the Ford colors and some members here have bought from them with good results.
I'm sure most are looking for more wire than you are, but maybe their minimums are not that onerous.
On a side note, that wire is the business end of a resistor wire, so you really should not run your Pertronix distributor module from it. They want to see 12v rather than the 7-9v that you typically see at the coil.
So if you're forced to buy a few feet of it, you can just run it all the way to the ignition switch and splice it in there, before the resistor wire.
paul