I doubt there´s no shop in Düsseldorf selling them.
It´s just difficult to find out ...

Btw.:
Looking at Rewe was not a bad idea but each branch has the ability to adjust their assortment to the "local taste" within a certain range.
So, a different branch may well offer Kägi Fret !
(I live near Heidelberg and our local Rewe has Kägi Fret availlable ...)
And despite Rewe markets in Düsseldorf get their supply from a different CDC I´m pretty sure Kägi Fret is availlable there !

Maybe you should check different branches (the larger ones preferably) or ultimately contact Rewe by Email.