The only dimensional difference between a conventional car's normal and "donut" spare is the width. The i3 tire width is already narrow, so a separate limited duty spare wheel and tire assembly would make little sense. A full duty spare can operate at much greater speeds, and for much greater distances.
You can get a mounted spare wheel and tire assembly from TireRack for as little as $364 plus shipping. I paid ~$505 for mine, which included an $80 TPM sensor (unnecessary for most folks - I added it because I occasionally take long trips in the i3) and shipping.
It will not fit in the trunk, so here's a suggestion for conveniently and safely carrying it, for those who do not regularly travel with more than 3 occupants in the car:
It fits the space behind the passenger seat perfectly with the seat slid all the way back, and comfortably reclined:
It does not interfere with lowering the seat back completely flat:
To secure it from being a dangerous projectile in an accident, give the front edge of the rear seat bench a gentle vertical tug, exposing its attach point:
Guide a strong rope around the steel support member:
Tie the rope securely around spokes at the level of the attach point. Hint - if you tie it reasonably securely before replacing the seat, it snugs everything up nicely as you push the seat bench back into position:
The BMW jack and wrench kit includes a chock and even a torque wrench (
http://www.ecstuning.com/Search/SiteSearch/71102182448/ES2530819/). It fits perfectly in the width of the frunk: