Shopping in Paris is one of the great free activities, that is unless you buy. Flea markets, fashion, books, artists, and more; a great outline of all the shopping you could possibly dream of, and more, can be found in the Frommer's Guide, either the Paris or France edition.
Another great website is Discover France, where you will find individual directories to booksellers (librairies), covered markets (march couvert), outdoor markets (marche en plein-air), and department stores (magasins), or specialty shops, including cheese shops (fromageries), chocolate (chocolatiers), and the famous Paris bakery (boulangeries).
Kaboodle shows Paris by District and Arrondiesment. It also gives some links to activities to do while in Paris.
The best way to figure out where to shop is to go to Frommer's Guide on Shopping. Then check out the map to find which arronidement the shopping is in.
Will and I are staying in the Latin Quarter, 6. On the Rue du Bac behind the wealthy residential area is shopping for the very rich. The Galerie Adrien Maeght with posters starting at 4.6 euros, is at 42 Rue du Bac. The left Bank is also famous for its booksellers. Choices include: Tea and Tattered Pages, 24 rue Mayer; The Village Voice, 6 rue Princesse; and, just over the line in arrondisement 5 is Shakespeare and Company, rue de l'Odeon (home to Sylvia Beach, "mother of the lost generation").
Much of the great shopping is located in and around the Avenue De Champs d'Lycee in arrondisement 8 (8e). Anna Lowe, 104 rue du Faubourg Ste Honore, for women's fashions. Slightly further away in 9e, along Boulevard Haussmann, Pigalle, are the large department stores of Au Printemps and Galeries Layfayette.
Antiques, check out Village St-Paul, 23-27 rue St-Paul in 4e. You are close to the Marche aux Fleurs, place Louis-Lepine on the Ile de la Cite.
The Paris Art Market is at the foot of the Montparnasse Tower in 14e.
The best flea market is Marche aux Puces de Clingancourt, avenue de la Porte de Cligancourt, in a suburb north of 18e. So, make it a day trip with a visit to the famous area of Montmarte where Toulouse Latrec wandered about.
There is so much more, but isn't this good enough for starters?