In the UK we can get the 453 spacesaver tyre easy as it is available on Ebay UK, and also UK tyre shops. I am sure you should be able to get a spacesaver in the USA/Canada. I have bought mine from Ebay UK for £100, picture links below....Max speed of spacesaver is 50MPH and is temporary use only.
https://i.imgur.com/5rfvIUs.jpg
https://i.imgur.com/sqBTq8S.jpg
Installation, here are some pictures of my Fortwo, note it is RHD, so seats are other way around
Spacesaver wedged in behind the passenger seat, I bought a cover off Ebay to make it look neat...
Here is a view looking down to the handbrake between the seats...
VERY IMPORTANT...ensure for safety, that the spacesaver is tied down with nylon rope to the end of the seat runners....
Passenger Seat belt buckle just lies on the spacesaver....
Note that with this installation, the boot (trunk) is not affected and has full space available. The passenger seat will be slightly more forward than the drivers seat due to the spacesaver behind (about two or three inches). My wheel brace and jack is in a sports bag behind the drivers seat and is tied down. It also has scissors to cut the nylon rope when needed. Finally the spacesaver has to sit on top of a 2 inch high bump which is under the carpet, to remain flush with seat back and the small rear bulkhead. This bump is part of the metal floor pan used to allow the passenger side handbrake cable to exit underneath the cabin and go to the rear wheel. It is not affected by the spacesaver on top of the bump. Cheers Colin.