Hi umamahesh,
Look for one of
these stores, I believe they are the German equivalent of the "Dollar Store" you'd find here in the states.
Good luck!
Here is the Pfenningland in Duisburg, with directions from Dusseldorf. I couldn't get the directions to start from the airport, but you can see the airport on the map. It's that gray spot about 5 km north of the start point. Hope it helps.
Link