This is so late as to be useless but may help for futur visits to France.

There are at least two magazines dedicated to model trains in France. RMF and Loco-Revue. Each comes out every month. There is very little on Marklin. I suspect DC is the order of the day in France. I find them at news stands in railway stations, and sometimes airports. Even if you cannot read French, the pictures are very good.

At the back is the section "petits annonces" or "petites-annonces/repères". Dates and locations of shows, swap meets, etc. are given I. Often they are by Departments, ie. 30 is Gard, Nimes area, 75 is Paris. Likely most hotels have someone who can translate the ad into English for you, and tell you which departments are close to where you are.

There are other fascinating magazines, Historail is about the real thing, concentrating on the history. Ferrovissime is about the real thing, but more recent, ie the past twenty years, and the present.

I apologize for not being quick enough to help with this visit. Hopefully this will give you a lead for your next visit.

If you can get to Mulhouse I strongly recommend Le cité du train. Il is the finest rail museum I have yet seen. ( I have not made it to York yet)