The federal states of the German Empire 1871-1920 – Flags in Lexicon and Shop https://www.flaggenlexikon.de/fdtlhirl_me.htm
The German Empire was a federal state. Between 1871 and 1920 it consisted of 26 federal states, mostly partially sovereign monarchies. You can order their flags on these pages.
• Duchy of Brunswick • Free and Hanseatic Town Bremen