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Sorry for the late entry into this topic which is of course 'my area of expertise', but have just returned from my summer holiday in Switzerland.
I no longer make 'special' trips abroad for French N in shops. Personally, I honestly believe it is now more economical to buy French N gauge on line from France (eg Trains 160), Germany (for the mainstream German made stuff eg Minitrix), Spain or Basar Valira (eg for the Mabar railcars) see Basar Valira in Andorra
However, if one is visiting cities for other reasons it is obviously worth a look at the model shops.
Transmondia is the 'home' of French N gauge with their own products etc. Back in the days when there were very few French N gauge items (late 70s early 80s) Transmondia was vital to us very few longstanding French N gaugers for their resin kits , but nowadays with 'mainstream' products such as the excellent recent Minitrix and Piko products, they are not necessarily the cheapest option and most of the many many model railway shops in Paris now carry N gauge stuff.
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