And to answer the original poster:
Some people put a spare front tire/wheel standing up behind the driver's seat. The front tire/wheel can do double-duty as a rear wheel.
You can get same-size aftermarket wheels from tirerack.com for a reasonable price, and they'll mount a OEM tire for you.
Of course, you'd still need to be able to jack up the car, and remove the lug nuts. See evilution's website for details on jacking up a smart:
Evilution - Smart Car Encyclopaedia