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It has been over 10 years since I lived in Berlin. However, as of 1992, there were not any music stores that stocked more than 1 or 2 tubas. Maybe this has changed.

There is a great museum on Tiergarten Strasse, the Musikinstrumenten Museum. They had several tubas, ophecleides, serpant, and other tuba relatives on display when I last went.

Berlin is a great city, have fun. I am hoping to get over there again soon.

I've got a letter from a shop in Berlin, called Der Blechbläser, where they offer me to buy tubas. The letter is now ten years old and I've no idea whether the shop still is alive.
However, the address is Freienwalderstrasse 25 and phone is +30-9765171.
Why not pay a visit to Hamburg and Gronitz? It's not too far away.
Address is Haydnstrasse 10, phone +40-891649
Good luck, Germany is an amazing country for tubas.
Lars