There are lots of asia shops all over Berlin. The nearest one to Haus Unger is located near Wittenbergplatz, one station with the U-Bahn from Nollendorfplatz. You can also walk there (16 minutes according to Google Maps) or take your bicycle (5 minutes according to Google Maps).
It is called Vinh-Loi (栄利市場 in chinese characters).
Vinh-Loi has a good variety of specialities from all the different asian countries. You will also find fresh vegetables you would not find in a german supermarket (such as thai basil or mung bean sprouts) and lots of imported frozen fish, crabs and squid.