Hawaii Surf is a cool skate shop in Paris. They have quite a selection of both longboards and skates (plus real surfs and rollers if you care). They also have some old school goodies including some mini Banzai board like this one.

Unfortunately, they don't however have those Madrid mimi longboards which you can see here and which seem to be only available in the states.

Added afterwards: I have recently been asked by a tourist nearby the place I live (ie. the center of Paris) about skate shops around; here are a few that I visited in person

In addition, there are a few things (very few indeed) inside the so-called Citadium department store nearby the other Grands Magasins (behind the Opera).

This being said, if you want anything other than clothes, you'd better head straight to Hawaii Surf which is the only serious option among these when it comes to hardware (decks, trucks, etc.; in particular, they are the only one to offer old school skateboards or longboards).