Since I suffer from sleep apnea, the first stop is always a long quest to buy distilled water for the humidifier of my device. In Berlin it was fairly easy. I went to a supermarket chain called EDEKA which are everywhere in Berlin.
I was in a mid sized store in an East Berlin neighborhood and after looking all over for it I had to ask for help. The woman helping me took me out the an alcove just inside the front door which had numerous seasonal items, ice packs and outdoor supplies. They had a one liter bottle and a larger bottle. It was pretty reasonable in price.
Watch out for so called “ironing water” as that can be distilled water with scent added.