Leave the hotel early on weekend mornings and take the metro to the far left bank outpost of Vanves, then walk a few blocks to Avenue Marc Sangnier and Avenue Georges Lafenestre for this small, excellent market, also an open secret among dealers. (Rumor has it that after dealers shop here, they bring their treasures to the far larger Porte de Clignancourt later in the day.) The tables brim with vintage boxes, glassware, old Parisian periodicals, posters, and other souvenir-ready material. Prices are congenial, but remember to bring cash—the ATM machine is a bit of a hike.
For more information, see the Paris Destination Guide.