Suitable space-saver spares are no longer being made except maybe by Vredelstein in Europe. The OEM takes a 14" x 6" Ford pattern steel wheel that barely clears stock front brakes & uses standard car open-top wheel lugs. An OEM space saver spare simply won't fit over big brakes on either end. When I inquired at some tire shops for a replacement, the young clerks didn't know what I was talking about. Any you locate may be up to 20 yrs old even if completely unused. But since they are/were only to be used once, at 50 mph max for up to an hour, maybe that's not significant. I had to use mine one time (cracked rear wheel), and in 60 slow, gentle miles, the spare was as smooth as a racing slick.
Drawbacks: 1)- when the factory space saver is fully inflated, it does NOT match the OD of a regular rear tire, so there WILL be LSD clutch wear inside the ZF. On the front, be prepared for some odd steering effects. Wet pavement will be particularly interesting on either end as tread is almost non-existent. They are essentially folding-sidewall 2-ply inner tubes with a printed tread!
2)- they require a gas bottle inflator and many aftermarket inflators use cheap PROPANE as a propellant! If the can says- Danger-Combustible, it means you are holding a hand grenade! At high temps combined with high altitudes, they have 'cooked off' and damaged front or rear trunks as a result of the explosion. Find one filled with a non-combustible gas! An on-board 12v compressor is a far better choice, even if not period-correct. Some tall, thin high-pressure spares may fit in the front trunk if you rearrange everything and drop the battery below the floor. It will NOT fit in the rear trunk space made for a space saver but only as a loose, bulky package. Such could fit better in a custom made fiberglas trunk insert.
3)- A flat 225-50 x 15" nor a flat 275-55 x 15" tire on a Campy mag will NOT fit in a completely empty rear trunk. Larger wheels & tires will only be worse. The flat must be carried in the passenger seat (or in your unhappy passenger's lap). Alternatively, carry a bungee cord of suitable length, put the flat in the trunk and drive with the lid unlatched, held by the bungee. The inside mirror will be blocked.