You´re too late. In the early 90´s there where 3 shops in Nuremberg, Hoffmann, Kebbel and Schiller, who sold Heinrichsen, too.
There are interesting flat museums in Goslar and Leipzig-Dölitz with a lot of dioramas. But they are not so large like the Plassenburg.
The best way to buy flats is to visit the Kulmbacher Zinnfigurenbörse 7.8.-9.8.2015.
The old collectors die, their collections are sold on ebay und the young people are more interested in computers & Co. So there are only a few museum-shops left.
Best regards
Michael