i would guess maybe OBI might sell this type of product or even a grocery store?
Glad to be of help :)
Carrefour or Auchan (the latter are usually cheaper) should have the Duck product, and Spolem will also have the coffee machine/kettle descaler which might be in a sachet.
Me personally, I use vinegar (ocet) as I hope it's better for the environment.
wikihow.com/Clean-the-Showerhead-with-Vinegar