You have several options:
A. Order a Cooper, they come with spares in the US.
B. 20mph is not the speed you'd have to drive on with a 'semi inflated' run flat, its 50 mph 'theoretically' but if its semi inflated, you can go hundreds of miles, with no inflation at all, you can go 90 if I recall. Perhaps someone else will have the chart thats in the owner's manual handy.
C. There is an option for a spare, the boot will fit one if you're worried for a long trip.
D. I can't imagine MINI bothering to put one on as other people have said, the technology besides being expensive, and heavy really works well.