You should be able to use which ever search engine that is used
in France to locate some dealers.  Jewelry suppliers in the USA
and England generally have brass, copper and other metal in
addition to the noble metals.  This is a good source for almost
anything, anywhere:

http://www.thomasglobal.com/search/heading.asp?new=Y&t=&keywords=brass%20sheet&country=FRA&hid=261340

And this is the US site: http://www.thomasregister.com/

Locate home machinist sites, model engineer sites, both steam
and rail.

Check out your local hobby shops.  They have racks of small sized
bar stock and sheet stock.  Lastly, industrial suppliers in your
phone book: look under metals, new and used.  I forgot -- some
junk/scrap yards and recyclers sell the wares.

Been there, found that the hunt can sometimes be more fun than
finding it!

Glen
Glen R. Perye Jr.

 [ The French terms are tôle de cuivre (sheeting of copper) and
 [ tôle de laiton (brass sheeting).  -- Robbie