Concierge.com's insider take:
Paris is crammed with antique shops, otherwise called brocantes or antiquaires, often full of marvelous assortments of objects. One concentration of shops is in the Carré Rive Gauche, literally a square within Saint-Germain of associated dealers, marked by blue banners; its four sides are formed by the Rue des Saints-Pères, the Rue Jacob (which becomes the Rue de l'Université), the Quai Voltaire, and the Rue de Beaune. The best place to start is a chat with Madame Fortier, who has been running Edouard de la Marque at 2 Rue des Saints-Pères since 1945, and is what the French like to call an oiseau du quartier—someone who knows everyone and everything. Edouard de la Marque specializes in accessible early-to-mid-century furniture (33-1-42-60-71-62). The Rue de Lille has several specialists in Art Deco and mid-century; the Quai Voltaire takes a wide swath, from medieval religious statues to dainty Venetian glass. Down the Rue de Beaune, several curiosités offer tempting displays of oddities (stuffed birds, globes, and miniature beds); a specialist in medical paraphernalia, including skeleton models, prints, and early texts, can be found on the Rue Jacob. Best to look out for the yearly vernissage, or opening, in June, where all participating shops stay open till midnight, put up their best pieces, and serve Champagne.