Well I can only relate to my experience of them, but all on 3 Series - my Z4 doesn't go out in winter weather!
I had 205/50 x 16 Michelin Alpin winters on my E46 and they were fine in colder damp conditions, but over the 3 winters I had it there wasn't any snow! Because it was an E46 they weren't run-flats because it didn't have any sort of Tyre Pressure Warning System.
Then I had run-flat Bridgestone Blizzaks (also 205/50 x 16) on the 325i I bought a week before the Beast from the East and they were just brilliant even in the snow.
Now I've got 225/45 x 17 Hankook I Cept Evo run-flats on my current E90 and they seem OK so far, although no snow down here yet.
Personally I'd probably stick to run-flats - I can't imagine how miserable it would be trying to change a wheel when it's cold, wet, possibly dark and there may even be now on the ground!
I can't suggest where to get them (unless someone is selling a set on here) but the size you want may have a bearing on brand options and prices. Where you expect to be using the car may also be a factor - we don't often get really bad winter conditions in most of the UK.