Planning a weeks trip to Paris, focused on locating Art Nouveau antiques (furniture, mirrors, lamps etc) for purchase. I know of the flea market in Saint-Ouen and plan to visit it. Are there any more suitable markets/shops like this in the vicinity (driveable day-trip) of Paris?
I do not know about specific Art Nouveau-selling shops or flea markets. You may want to target antiques in general, "antiquités", "brocante", "antiquaire" in French.
About flea markets: these occur in Paris on a regular basis, at different places. These are commonly called "vide-grenier", literally attic-emptier, the same as US garage sales, but grouped together like a market. You can find anything, including some retro furniture.
The following website in French is an directory of the upcoming vide-greniers: http://vide-greniers.org/agendaDepartement.php?departement=75
There should be a magasin for sale at almost every kiosk listing all brocantes, puces and the like. I forgot the name, but I got my first sample by just asking for it in at the local kiosk.
You might also want to check Stade Charlety. If I recall correct, there should be huge market there once a month.
Further more I would recommend the brocante of porte de vanves on Saturday and Sunday. Make sure to be early to find the nice pieces. The latter is not as big as the one in Saint Ouen, but certainly worth a visit