The wheel you're referring to is the base steering wheel used only in '71 on the Mustang. Often called a Boss wheel since many Boss cars were ordered bare-bones as possible to keep the weight and cost down. No Mercury vehicles were equipped with this wheel. Ford also used this style wheel in different colors on base '71-77 Mavericks and '71-80 Pintos, basically "Throw Away" cars. Other than powertrain, not a lot was salvaged when these cars were at the end of their life cycle, making finding one in decent condition difficult. Although a base steering wheel, when the word Boss is used in the description, sellers' eyes gloss over with $$ signs, and realistic prices go out the window. I haven't seen one of these wheels for sale in quite some time. The last time I did see a NOS wheel for sale, it was offered by a seller notorious for insane prices. He used "Boss" in the description, which immediately signaled a price not friendly to us mere mortals. Although there are probably none to be found, I'll list the Ford part number for the black wheel for reference if you want to search some online sources. The original part # is D1FZ-3600-E, replaced by D2FZ-3600-R, with a final replacement of D7FZ-3600-D. The D7FZ version was discontinued in 1995, so you can see it's been a while since Ford had these wheels in their parts system.
I would try Don @ Ohio Mustang Supply or Mike @ Motor City Mustang to see if they might have any of this type of steering wheel looking for a new home. Unless you specifically want to use the basic wheel, they might even have a used Deluxe two-spoke wheel for a deal. Don (OMS) also now has the repo two spoke deluxe wheel, pad, and horn switch.
Good luck with your search.