would any 125/70/18 wheel fit?
No.
The bolt pattern (PCD) must be 5x112 - anything different and the bolt holes in the space saver won’t line up with the bolt holes in the car’s wheel hub assembly so it won’t bepossible to fit the space saver on the car.
Have you spoken to your dealer’s parts department to quote you a price for a new space saver plus other parts that would form part of the space saver kit for your car?
You could also try forum sponsors
@COX Motor Parts.
If you’re happy to have a used space saver, they do come up for sale on sites such as eBay. You could also try car breakers in your area.