Hello Chris,
If you are looking for driver-quality rear bumper guards, check with Don @ Ohio Mustang Supply (OMS) or Mike @ MotorCity Mustang. They are both members/vendors here and have parts cars.
The 71-72 rear bumper guards (D1ZZ-17984-A) were available separately or in a kit form (D1ZZ-17984-B) from Ford. The kit contained the guards, pads, mounting hardware, and installation sketch. The individual guards (D1ZZ-A) were discontinued 3/81, and the kit (D1ZZ-B) 5/75, so they have been out of the Ford parts system for quite a while. A company that sells NOS Ford parts shows stock on the kit.
http://www.greensalescompany.com Type D1ZZ-17984-B where it states, "Check A Part Number Here." Type the part number without the hyphens, or their system will not recognize the number. I don't know how proud they might be of this particular part; like other parts sources with vintage Mustang parts, sometimes they get a little crazy with their prices. Won't hurt to call them, as they show five sets of the guard kit. And you can always search for the D1ZZ-17984-A individual guards if their prices hurt your feelings!
There is an eBay listing for two NOS individual guards (D1ZZ-17984-A), but it is a bid auction, not a buy-it-now. The starting bid price.....$299.99 + 15.95 shipping! I understand supply and demand as well as anyone, but $300.00 + is a handsome profit for a part sold for $ 21.65 each. Just adding a little perspective!
Good luck with your search Chris.