I second the voice of loranger.
IMO Photostock is the best place (lowest prices, nice and competent people) to purchase films in Paris.
Négatif plus is the cheapest for development but they don't value their customers as much as professional labs. Once I lost my ticket and got into some trouble getting my films back. And they don't excuse when your films are scratched occasionally. In professional labs they know the face of every customer and it's quite easy to give the technician special instructions: for example, if you want the development to be supervised manually, which is extremely useful in case you forget the speed at which you shot the film. Sometimes you have choices over developers as well. For example, Xtol, Microdol X or Id11 (if my memory is good. Microdol X should be gone now.) for Publimod in the Marais. If the price is important, try Processus (another professional lab) in rue de la roquette near Père la chaise. It costs about 6 or 7EUR (around 4EUR for Négatif plus I believe) for development only, E6 being more expansive. They use TMAX developer but I have had nothing but good experience with them.
And of course, enjoy Paris.