You could get a doughnut spare from a Gen II, or maybe from a Dakota/Ram. Other makes of truck perhaps. Not much else out there with 6 lug rims on non-truck vehicles.
Edit: I don't know if a gen 2 doughnut would clear the brakes on a gen 3.
While I have a doughnut in my gen 2, I also carry a puncture repair kit & small air pump. I'd only use a doughnut if the tire can't be plugged, and then only in emergency. Two tires of different diameter on the rear for any length of time would probably be a more expensive fix than a tow.