Hi Tony
There's a place called Euromaquette which is very near to the Gare de Paris-Lyon which always takes out full page adverts in the international Tamiya Model Magazine. The address is Galerie marchande Gamma, 193 rue de Bercy. If you go to their website at
www.euro-maquette.eu you should find a map. But do check out things like French lunchtime and Monday closure.
I have visited once or twice when they were at a different location and there is lots of stuff in display cases, but from my perspective hardly anything on WW1.
I've also seen a box advert in the same magazine for EOL International Hobby at 3 rue de Louvre which is open from Monday to Saturday from 10 until 7pm (Monday 1 until 7pm). The nearest Metro station is Louvre Rivoli. The ad isn't too inspiring but you never know.
Best wishes
Nigel