Your cheapest option’s probably going to be a junkyard. Lots of Subarus have compatible wheels with the same bolt pattern (5x100) & bore size (56.1 mm). For about $12, I got a spare tire from a Subaru Legacy sedan, including the bolt & washer to hold it down.
Try to find a spare tire in T135/80R16 size. It’s an almost-perfect match to the car’s stock tire circumference. Some Subarus, like Foresters and most Outbacks, have compatible
wheels but slightly larger
tires. If you mount a spare on a driven wheel — the rears in our case — the greater the difference in size between it and the stock tire, the more the differential’s going to spin while you’re driving …and that’s not great for it.
A couple of resources to help you cross-reference:
To compare your tire size to potential spares, checking the difference between them:
Tire Size Comparison
To look up wheel specs like bolt pattern, bore, & offset for cars by make/model/year, which you can hopefully cross-reference against a junkyard’s inventory to identify potential donor cars:
A reference guide to wheel and tire sizes for your car