hi, just to add some more i discovered today: the bookstore Epsilon, 33, rue de Vaugirard in the 6th is full of both grk and latin books and is worth checking out.
for rare classics books, also try Picard at 82, rue Bonaparte in the 6th, and the stores on the same street. just bought from there today the only complete latin monoling dictionary i have ever seen.
also, of the Touzot bookstores, the one which is most dedicated to classics books (including some beautiful ones: e.g. an early 1500s cicero with handpainted capitals at the start of each oration) is the one on 19, rue de Seine in the 6th. cheers