gourmet/small food stores
A must for us is G. Detou on rue de Tiquetonne between rues Montorguiel and Montmartre. Buy vanilla beans, silver dragees, cepes, chocolate, caramels, veal fond, chestnut ligueur, escargots, confitures, you get the point.
On rue Montorguiel, there is a lovely "bean man" who sells beans and seeds as well as spices in bulk. I find it convenient to bring small zip-loc baggies.
Of course, there is Maille at the Madelaine where you can often find a mustard pot to match your china pattern or bring your own jar to be refilled as well as buying their many flavors off the shelf.
Basque products at Pierre Oteiza on Blvd St. Michel not far from the quai.
You might try searching for specific food products on this board since shopping is often discussed..