I heard a BMW driving instructor say "don't buy snow tires from BMW, BMW sells pretty snow tires, you need real snow tires." Then he said he drives Nokian Hakkapelittaa's.
Tirerack.com X 1000! If you are looking for both wheels and tires, Tirerack prices are hard to beat. The website automatically only allows you to choose wheels and tires that will fit your car. If you go to the wheel and tires forum, you should find a guy named Gary. He can answer almost any BMW tire or wheel question.
I bought my winter wheels (and first snow tires) from them. Tirerack house brand wheels, Sport Edition, are decent quality and cheap. Light too (22 lbs). Shipped to your door in about a week.