In fact, in Paris, you have 2 main possibilities...
Or you take the official supplier of straps for JLC, which is Camille Fournet, and you will even have the possibility to personnalize your strap,
Or you contact " L'Atelier du Bracelet Parisien ", wich is very near the Place Vendome...and like Camille fournet , you will have the choice for the leather and the quality of the finish...
You have a third solution with " L'Atelier Jean Rousseau "...
Hope this help!
Nicolas