Weekends are the times to find pop-up flea/antique markets (brocantes) all over Paris. Posters occasionally point the way but even better is the website Vide-greniers.org. It’s in French but pretty simple to navigate. Links to location maps are included.
Be sure to check out the big vintage show this weekend: Salon du Vintage, http://salonduvintage.com, entry 4 euros, Espace d’animation des Blancs Manteaux, 48 rue Vielle du Temple, metro: Saint Paul/Hotel de Ville