I'm afraid there is less and less shops on Beaumarchais boulevard who sell films, it's preferable to follow the thread given by ajmiller, you will find a rather up-to-date list for shopping film.

When I was a Parisian, a good place to see musicians was the underground stations, I don't know if it's true again. See to the large interconnecting stations, like Chatelet-Les Halles, Charles-de-Gaulle.

Have a nice stay,