The airport and the main train station have many places to shop. Rossmans at the main station has almost anything you need and at good prices. 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. On the underground level across from Rossmans is a smaller Asian store that is also open on Sundays.
Information about Sunday Shopping in all of the German Länder