on track with what
@Gary Stolt suggested, especially with 21" rims, and if you have the space, a set of smaller rims and snows can run cheaper than large rims and snows.
Tire Rack sells em, I don't think Discount Tire is affiliated with Tire Rack but in any case, DT has a wide set of rims. Belle Tire, I think even Sams and Costco sells rims.
Bought a set of used rims for my truck years ago, seller bought up to 20" and dumped his standard (at the time)17 rims. paid under $40 each.
Needed a rim for the Explorer this spring, junk yard "ugly" (chrome rim with Al crude) ran me $55, painted it and the others basic black and they came out decent.
Anyhow, spare rims can be cheap and save the hassle of remounting tires in spring/fall