The first time I came to Paris, I thought the vintage shopping was terrible. Dirty and tattered clothing at prices that were too high. Since then, I've found the right places to go with the help of some recommendations and the Internet.
Le Marché Au Puces quite literally means the market of fleas. There are two major flea markets in Paris; Saint Ouen, also known as Clignacourt, and Vanves. Saint Ouen is open on Saturday, Sunday, and Monday of every week. I went on a Monday around 2 pm, and a lot was still opening. Some vendors never opened, and most closed up shop between 5 pm and 6 pm.
I used to take the subway to the stop at Porte de Clignacourt, but realized that Garibaldi is actually closer. If you take the Clignacourt stop, you will have to walk through, what I would consider, a pretty sketchy market that has knock off cell phone cases and over bleached jeans. There are signs pointing to the direction of the flea market everywhere, so really there should be no reason to get lost.