The airport has many stores where you can shop, and usually the train station does too, but it is being renovated right now and only the stores near the platforms are open..
A small REWE on the Kaiserstrasse is open Sundays from 08:00-22:00, other days from 8-24:00. Their selection is pretty extensive for a small store.
The large Asian store, Gutleut Asiamarkt located at Kaiserstrasse 72, is open 12-16:00.
Information about Sunday Shopping in all of the German Länder