You can buy temporary "donut" spare tires directly from Ford or other automakers' official parts departments, from tire manufacturers such as Goodyear, or from retailers that sell several brands, such as Tire Rack. You need to know the proper size and bolt pattern for your vehicle.
While they are only meant to be driven on for a few miles in emergency situations, a compact "donut" temporary spare tire saves space and weight in the trunk and is less expensive than a full-size spare tire. Consult your vehicle owners manual to find the proper size of donut spare. Size is listed as the tire class, sidewall width, height, performance rating and diameter; for example: T135/80R18.
Buying directly from an automaker costs more but insures a compact spare that is the proper size and allows for trouble-free operation. Spending extra on a "high-performance" donut is a waste of money, as these tires are only meant for very short distances.