If you are a vegetarian, I would think you would prefer the roving markets that happen on a regular weekly schedule in all of Paris. Additionally, the large everyday market ---Marche Aligre(metro Ledru-Rollin) has an astonishing variety at very good prices every day except Monday and an indoor market with two good cheese merchants, one of whom sells beans, grains, nuts, etc. There is also a very small, but quite good grananry on the other side of the market.

Since I prefer diet Pepsi to diet coke(no comments, please) I usually buy the two liter bottles from Franprix or G20. Both have an amazing variety of yoghurt and dairy products, if you are not vegan.