I found it on "Kleinanzeigen". It's a website similar to "Craigslist". The seller is a musician himself (keybords) and had a studio in which he used the neon sign as decoration. He had acquired it many years ago when a music store closed down and now wanted to get rid of it.
I had to pick it up in person, because he did not want to ship it for understandable reasons. I didn't even know that such a thing existed.