I bought a set of factory 15" steel wheels for my SVT for use last winter. I would have gone 'aftermarket' for the steels, but apparently, the Canadian market for this fitment is not very big, and they were priced at $130 CDN. I paid about $80 CDN at the Ford dealer.
If you're in the US, Luke at tirerack.com will give you a better deal than that; if you're in Canada, after exchange and shipping is taken into account, the dealer price is better.
Anyway, the 15" wheels worked great....the key thing is tire size. Get something of the same circumference to maintain ride height and speedometer calibration. My snows are 195/65-15. They are "Nordic Icetrak" from Canadian Tire, and I'm quite happy with them. Tread pattern is sort of a cross between the Michelin Arctic Alpin and the Bridgestone Blizzak.