I picked up a leaflet for an Expo-Bourse Playmobil in Bastogne (in Belgium, right across the French border, a three hour drive from Paris),
Saturday 29th November and Sunday 30th November.
I have no idea if it's a full-blown fair with sellers or if it's just a diorama about the Battle of the Ardennes, though.
Now, if you'd be in France
the 1st or 2nd of November, there's a Playmobil fair in Templeuve in France, a two hour drive from Paris. I've been to that one, it's the real deal
Maybe you should take a look at this page on a French forum called Playmo'Délisme, it lists all the upcoming Playmobil events near France, I believe:
http://www.playmodelisme.com/f163-EVENEMENTS-EN-2014-br.htmFor buying Playmobil, I don't know the specifics for France. If it's anything like Belgium there, and I'm pretty sure it is, you can find Playmobil in lots of large chain supermarkets (Carrefour, Colruyt) and basically every toy store.
If you'd be willing to drive to Antwerp (Belgium), a 3,5 hour drive from Paris, there's a toy store where you can find older sets - and plenty of places that carry the entire range of sets available now.