Are you specifically after BMW clothing?
If not, there's a large selection of bike shops not far from the main Hauptbahnhof / train station, with a much bigger choice than BMW & at good prices.
200m from Hammerbrook station is Suderstrasse. There you'll find a large Louis shop & another recently opened Dianese stockist across the road.
About 400m north of Suderstrasse & running parallel is Nordkanalstrasse, here you'll find Polo & Hein Gericke plus one or two other independent shops.
I've been to all, HG & Louis are the best. If going to Louis, make sure you visit the other building to the rear, set on several floors. The 1st & 2nd floors are full of bargains.
Touratech now have a retail outlet a few km north on Lubeckerstrasse.