The one I always visited was in the outskirts near Rudow but unfortunately it has now closed down. You are correct in thinking the fabrics are mostly US at approx $30/meter Cdn.... BUT ... you may be able to find shops that sell "Westfalen" fabrics made for the traditional costumes of that area that are 100% cotton.... Also you could check with shops that sell draperies as some carry fabrics as well. Do you speak German? If you do... you can google patchwork shops in Berlin and find a few other sites to check out. There was a shop near the TV tower that was in East Berlin that sold all sorts of fabrics.... it was overwhelming but good to see.
http://www.berlinmodernquiltguild.de/
http://berlinquilter.blogspot.ca/