At least in the USA, buying a winter package ready to bolt on is quite a bit less expensive than buying it in parts...here, just buying a wheel costs more than a pre-installed tire/rim/TPMS with center cap!
But, BMW only sells the i3 winter tires premounted on the 427 wheel here...if you wanted to use say the 428 wheel, your costs would skyrocket since you'd not be getting the package price, and have to buy the bits separately.
I do not know if this is a BMWUSA thing, or it's available elsewhere...but, see if the local dealer has a special price for a package rather than just the parts.
In the USA, the 427 wheel/tire retails for $431.76 (part number 36-11-2-357-088) as of a few weeks ago, and they just started to ship them in the last week (or at least they said they would be). I should have mine in at most a couple of weeks. I'd have rather had the 428 wheels, but the price difference was not worth it to me at all.
One big supplier of tires in the USA - www.tirerack.com, says they will have a compatible non-oem wheel available later this month, and have had the tires and TPMS available for at least a few weeks. Often, they are the cheapest, but not this time with BMW's package pricing.
FOr those that have the i3 on the optional 20" wheels, those are summer performance tires, and will not stop, corner, nor handle great when the temperature drops compared to the winter tire. If you live where it gets cold...better safe than sorry. THe 19" stock wheels use an all-season, but even then, in the cold and wet, the winter tire package will be safer.