Congratulations on a wonderful visitor experience.
At a recent visit I was surprised that there are not many railway locos and rolling stock for sale in your shop. I suggest that MiWuLa would be the perfect place for a well stocked model railway shop selling models from Europe and the US to mirror the MiWuLA zones. Maybe you don't want to compete with other Hamburg model shops? Why not a local map showing visitors where they might buy things?
I agree with your idea. But you are right in assuming the independence and non-affiliation that has been the branding policy of the Miwula management for years. Miniatur Wunderland is a landmark in Hamburg of its own.
However, as far as I remember from my visits, a little flyer of one of the interesting shops was available, but it must of course be a question of ad money - as by tradition model railway shops are not very innovative themselves.
Another fact is that only 5 - 10 per cent of visitors are model railway enthusiasts at their arrival in Miwula, so handling a large range of models in H0, N and Z would be a challenge, and a costly such. (Followed by guarantee obligations and retail responsibility etc.)
For your own choice, dealers in Germany and other countries in Europa may be found here:
Searching for Hamburg Area give in 20xxx, 21xxx and 22xxx as post codes in the search function. The German word for dealership search is Händlersuche