In Paris, there's Gibert (26-34 Boulevard Saint-Michel). It's a giant bookstore with several floors of books including one whole floor devoted to scientific books. A sizable part of that floor is dedicated to maths, and there are enough books that they're sorted by subject (number theory, algebra, analysis, ...). During Springer's yellow sale, there's a big table overflowing with cheap Springer books, including many tomes of Bourbaki. If you go, there's a high risk of running into somebody you know.