Search here and on FABO for cars being parted out. Might get lucky
try wrecking yards there is one here in northern CO that has many 60-70s mopar I'm sure some would have the remote mirror? and its a pick your part so for a few dollars you might get lucky. Might be a long drive for you:) depending on where you are.
See if you can find the manufacturer of the aftermarket remote mirror and contact them directly to see about getting a replacement nut and clip
Take mirror to an Ace type hwd store (the ones that have the trays of misc parts that Home Depot doesn't have) and see what you can find that will work. and make your own clip with the help of someone sending you a detailed photograph.