Not sure if there is a Hornbach hardware store near Baden (they are actually in Germany and Switzerland as far as I can say), and they have a pretty good selection of aquariums and fishes.
There is also a more specialized shop (probably better for you) in Villmergen (
http://www.aquarium-shop.ch/), which is about 25 Kms from Baden.
Finally, in some Qualipet shops (not sure if all) they have some limited selection of aquarium and fishes, but I frankly haven't bought anything from them yet.
Other than that, I haven't bought fishes crossing the border (in my case, the closest one is France), so I don't know the customs implications. I would suggest you to but the aquarium first, set it up with the gravel, plants and water, and in the following weekend to go and buy the fishes so you allow some time for the aquarium to stabilize chemically and biologically.
and Hope this helps, yopo