Want to buy a spare - but where?

Are you looking for a compact spare? If so, then eBay or any local junkyard is the best bet. Any mounted compact spare tire from Gen3 or Gen4 Prius will fit. There may be other cars that work as well such as some Corolla spare. You can also order new from a dealer, but it will cost you a lot. Instead, I just ordered a generic seel wheel from Walmart and had them put one of newish OEM tire on it when I had to replace a pair of OEM tires (one tire could not be repaired and they did not have the same OEM tire, so I had to put two new identical aftermarket tires. I keep mine in the footwell of the rear seat behind the driver-side.

 

I bought one from the dealer. Compact spare and wheel mounted $200ish. Beats rummaging or eBay crapshoot. Keep it covered in trunk except when I need trunk space then I put it in front of a rear seat.

 

Yap, that should work. 16" wheel for a temporally donuts tire 135/70/16.

 

Although it should fit on PRIME, it maybe bit smaller than ideal size. If you can find 135/80-16 tire size with 5x100 bolt pattern, then the overall diameter of the tire will be closer to the full size 195/65-15 tires.

Looking up 135/70-16 tire has diameter of 23.44" whereas 135/80-16 is 24.5". The OEM full size tire 195/65-15 is 24.98". Some Prius may have been fitted with a smaller temp tire to fit in the spare tire well, but for PRIME that is a moot point since you have no spare tire well available anyway.

Here is one with 135/80-16 tire.
2016 2017 2018 2019 2020 TOYOTA PRIUS SPARE TIRE WHEEL DONUT 16" 5X100 135-80-16 | eBay

 

Here is another with 135/80-16 tire. But, if you are going to buy a used temp tire from eBay, check what year model it came off. Even if it looks new, a tire older than 6 years are risky to use. If it is going to sit in your car for next few years unused, the temp tire may be too old to be safe when you really need it.
2010-2015 Toyota Prius Emergency Spare Tire Donut Wheel T135/80d16 | eBay

 

You strap that spare tire down, right? If someone hits you, you do not want that spare tire becoming an unguided missile launched inside your car.

 

You can search on Tire Rack (great resource), then shop local (with info from Tire Rack). Temp spares I would think tend to be a seldom-sold item, but they can order anything I'm sure.

 

I called Tire Rack and they were unable to give me much help when I was looking.

 

Just search for the temp spare size listed in the Owner's Manual. Either 3rd/4th gen. My 2 cents: 3rd gen spare size is better, closer to OEM tire OD.