They are hard to find.I got mine out of 1971 Mav so check them.And many MustangII's had then until about 1976or77.Also check Mustangs up to 1968.Or even Falcon's.Good luck it only took me 4 years to find one.
By pure stroke of luck I found one in a 68 Ranchero. The best thing I can tell you is learn to recognize an 8 inch and stick your head under everything FoMoCo you can find. Middle 60's to middle 70's. Worked for me.