Don't know if the seat is still available but I did confirm several things after the only piece of luggage missing on our flight was a booster seat for my son. The Welt was very helpful in finding a local merchant who carried a booster seat.
1. They do not sell booster seats in the Welt Store but DO sell full on car seats.
2. There is an OBI (Home Depot for Europe) about 1/4 mile north of the Welt. They sell safety vests AND booster seats. Here is the exact location:
2007 335i Monaco Blue 11/06 ED Black Forest-Lindau-Oberammergau-Salzburg
2010 535i Alpine White 10/09 ED Castelrotto-Lake Como-Grindelwald-Zermatt-Gruyeres-Luzern-Alpenzeller
2011 Porsche 911 Midnight Blue Carrera S PDK
2013 535i Carbon Black 07/13 ED Munich-Salzburg-Dolomites-Fussen-Alsace-Frankfut