Very true! But I myself live paycheck to paycheck like some on here and I know their daughter does too. Some idiot sales person talked her into the car with the SPECIAL package that had the 18" rims.
Actually it's the other way around. It's their job to up sell/push you into buying more stuff/options. The idiot is the one that says yes instead of sticking with they want and telling the sells person no.
Now to answer you question. You'll need to take car to the dealer or tire shop and have them test a 16 or 17 inch wheel to see if it works. That said most tire places have payment plans so you can make payments on the tires (regardless of the size you run). If it were me, I'd either get a cheap 18" steel wheel from the tire shop and just get studless snow tires on it. Or (if the has the tps) get a used factory set of wheels from the local junkyard and mount studless snow tires on them. This way I can change the set between summer and winter tires myself (but most tires places won't charge you for swapping a set of wheels if one of the sets of tires came from them).