My great grandfather, Georg Arndt, supposedly owned a women’s clothing store in Berlin named Arndt und Kluss (or Kluss und Arndt). He was last seen by his family on Kristallnacht and I have traced him to the concentration camp where he was killed but I can’t find any information about the store. Any ideas?
I would guess that the sources you are looking for are in German, and it might be necessary to travel to Germany to have a go for your research in German archives.
For a starting point, the Jewish museum of Berlin has an exposition themed around the Kristallnacht in Berlin, it may well be that they have some data in their archives.
They provide a starting point for family research on their website here: https://www.jmberlin.de/recherchen-zur-familiengeschichte
Maybe you could use the link to the adress listings of Berlin they provide: https://digital.zlb.de/viewer/berliner-adressbuecher/ maybe looking up the adress for family and business proir to 1938, maybe 1937 and with the address of the shop, you might find out more.
Then there has been a project of the Humboldt university in Berlin collecting data from Jewish shops during the Nazi time. The searchable database can be found here(The link featured by the Jewish museum is not working anymore): https://www2.hu-berlin.de/djgb/public/en/find
I really hope this helps you finding out more about your family history