In Paris now need Canon Camera battery

Carrefour Express. The closest is probably across the river about even with the cathedral, about two blocks in on R. Lagrange. If there's a closer 7-11 type joint (Monoprix, Franprix, et) stick your nose in there. Stop in any phone shop along the way.

There's a Darty at Republique, 1 avenue de la République which opens on Monday at 10:00 if you haven't found anything by then. Closed tomorrow of course.

Have you tried Googling a camera store?

I just did and found lots. http://www.google.com/search?q=camera%20store%2C%20paris%2C%20france&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&source=hp&channel=np

Any of the FNAC stores will have them. Just go to the camera department.

FNAC next to the Châtelet - Les Halles station where you probably got off the train from the airport. While there you can buy a pass to the Lourve and avoid the ticket line.