If I was to stop by in Stuttgart, I would definatly try to get to see Wolfgang Frey´s layout.
It´s a private layout not open to the public, and chances to get in would be slim at best, but you COULD try to write him a personal mail, telling him that you come all the way from the States.
(No harm in asking. The "NO" you already have! ;-)

Frey´s layout meassures 45*18 meters. It is an EXACT scale model of Stuttgart Hbf and surroundings.
Although N-scale, it leaves nothing to be desired in terms of details and realism.
"Impressive" doesnt really cover it!

I know he had some problems, and now his website seems to have disappeared (again). Anybody is in the know?

In the city center of Stuttgart is a large toy store. As far as I know, it has also model trains.

And generally for every German town:



Also pay attention to Muller department store in Germany. The bigger ones do sell Märklin, including spare parts. You will not find there the most expensive models, but the collection is not small either. Muller does produce from time to time special models that are real cheap. My 94254 passenger car was bought there. It has "Katja" advertising lettering and cost 14,99 Euro (The 4039 it is based on is more expensive). They do not sell Märklin only, but also model kits (Faller), ready built mountains (Noch) and all other things to make scenery on your layout. I spotted also Faller stonecast houses, if I am not wrong. If you are a starter with Märklin, Muller offers good value for money. I visited Muller in Würzburg, but watch out, they have 2 stores in the city center over there. You must visit the large one, that is the store with Märklin.

If you manage to get outside Stuttgart, there is the big marklin layout at Merklingen:
http://www.modellbahnshow.de/index.html