I hoped to find pretty much anything waldmeister here, but only found brause, soda, jello and saft. No ice cream yet
If it's not too far out of your way you could try "Eiszeit"
Müggenkampstraße 36
20257 Hamburg
I don't know how old this
article is as it has a slightly different address (same street different number than
this one), but it says that it probably has the best Waldmeistereis in the city. And I believe they call the flavor "forrest master", but it shouldn't be too hard to find out which the correct one is.
The problem is, that I could not find any confirmation if this ice cream shop even still exists, or if they fell victim to COVID like so many others.
Usually I would imagine that it should not be too difficult to find Waldmeistereis in Hamburg, but I have not really been to Hamburg, except for a couple of hours that I spent mainly around the main station, waiting for a replacement train, because the one I was on had broken down.
Anyway, I hope you will find success in your "Waldmeister-Hunt".