Le Sentier

The oldest textile neighborhood in Paris, north of the 3eme near Reaumur-Sebastopol.

Most of the stores do wholesale only ('vente en gros'),  but  some  indicate in the window that they do retail ('vente au detail'). You'll get some of the most fashionable items on the planet, hot off the press. It's definitely one of the best places for shoe lovers.

Au pied de Montmartre

At the foot of the Sacre-Coeur gardens, you have a whole paradise of discount fabric stores, stretching all the way to boulevard Rochechouard and the Anvers metro. The most famous spot is the big blue hulk of the Marche St Pierre, in operation for most of the 20th century at the Eastern corner of the park. You can also try Reine across the street.

Be aware that these stores run entirely on fabric closeouts that can sell in no time. Make up your mind and buy it immediately.

The smaller stores around also have great bargains.