Very uncommon size. With the correct offset and tire size, I'm sure it would fit fine. But there are not many 21" wheels OR tires on the market, so people opt for the more common (and usually less expensive) 20" wheels/tires. Good luck.
Looks like 285/30/21 would match the stock diameter pretty close, tirerack lists a few of those available.
MRR Wheels offers there FlowForged FS01 in a 21" option
is custom built specifically for the Mustang body, you can add a custom finish and shoot your own off sets. plus it wont cost you a arm and a leg, great option.
OEM Sizes are 265/35ZR20 & 285/35ZR20, almost here in Europe
The equivalent sizes on 21s are 265/30/21 (0.16% smaller) & 275/35/21 (just 2% bigger) or 275/30/21 (1.3% smaller)
Don't think that's ridiculous or weird most of the cars of today wear 1 inch bigger wheels than max oem sizes and no one is complaining about .... :shrug: