Batteries

You can try DM or Rossmann, which are somewhat like a CVS/Walgreens. They are all over Germany, so should be easy to find one near you.

Otherwise, there are specialist shops for hearing aids that could probably also help. One chain is called Kind, and they have locations throughout Germany.

You should also try Timi or an Apothecary.

Pretty sure that a Saturn or Media Markt will have them too.

There will be a Rossmann in more places.

Big dept stores like Galleria should have them, as well as electronic stores like Saturn.

Thanks to all for the speedy responses..