First off, it can be done, and you can find a ncie set. It may take some time to find a good looking set, so don't be in a rush.
Discount tire often has trade in rims, where they've upgraded some one and kept the old rims, or trade ins, you can ask them locally, go to their web site, or find their eBay auctions.
Newspaper ads, both used and new.
If you live near a large city, check out deals there.
Ask around, when you see a
stang with custom rims, I know lots of people have their old OEM set sitting on the side of the house, cuz they got 18" or what ever.