Yes.
If you need snow chains, you buy a set that fits the tire size of the vehicle.
the tire is located under the car in the back
No.
It is under the very rear of the car.
Underneath the very rear of the vehicle.
Snow tire chains can be purchased at almost any store that sells tires in an area that it snows. Walmart is one place that sells snow tire chains. It is also possible to find them online at any tire store.
under the back mat in the far back of the vehicle
Almost any major tire store will stock them.
Yes, it is a temporary (donut) spare located under the rear.
T125/70/15T125/70/15
On a USA vehicle, no.
Not possible