NEED: 215/70-16 Spare Tire

Make friends with someone at a small local tire shop and ask them to keep one for you.

First, take a good hard look at the condition and remaining tread of the tires that are on the car now. (Be honest with yourself.) If you have one or two that are getting kind of close to the wear bars, buy a new tire (or pair of tires) and put the best used one on as the spare.

That, or stop around to all the local tire shops (don't call). Tell them you want to buy a good used take-off tire in that size, and leave your business card with the tire size details written on the back side so they can pin it on the bulletin board.

Might take a few weeks for a set to come in, but it's in the tire shop's favor - they get the tire for free, and sell it to you for roughly $20 - $40 mounted, depending on the size.

I use take-off tires on my utility trailers that see very little road time - just like spare tires that never get used, they rot out long before they wear out. And the special trailer duty (ST) tires that allegedly resist rot better are a lot of money and usually special order only.

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At the store?

Why would you even consider a used tire for your Wife's car. Oh, I get it, it's your wife.

What else

You need to get a tire that is the same size as the tires on the ground, which I assume are the same as the tire tha tis on the spare tire carrier.