I stumbled on this custom belt shop in the Marais one day and have been back since for more photos. The second trip, I had to do a bit of negotiating with them but was in the end, allowed to take a few more. I have no idea how much their custom belts cost, but it's a lovely place to visit just to see the beautiful leather strips and exquisite buckles. The workbench caught my eye as well.
20 rue de Sévigné