After putting only 3,000 miles on my 328i and already getting stuck buying a new run flat ($260.00 with tax), I was determined to find a spare kit. It took a lot of searching. I think we have a conspiracy going on between BMW America and the tire companies who manufacture run flats. BMW's answer is that it's not available in the US. This kit should be readily available (it is in Europe) - but it's not.
Go to Bavarian Autosport: BMW Parts and BMW Accessories | Bavarian Autosport
search: wheels and tires> factory bmw wheels - scroll down that page and you will find the spare kit. Ordered mine today.
Cannot wait to get rid of these run flats. Now it's a shame to be wasting some of my limited trunk space - what were they thinking????