I've brought you to Losco's in the Marais, several times. They sell only fine leather belts and a wide assortment of fabulous buckles. I've been allowed photos in the past but this day Madame gave me a limit of two photos but I negotiated a little and got four. Ahhh the French. Regardless, you must stop in and if nothing else have a look at the workbench along the wall and breathe in the rich smell of leather.
Losco
20 rue de Sévigné
75004 Paris
Métro: Saint Paul