Tucked quietly off the main street of Rue des Francs-Bourgeois, 
this large mercerie has a wonderful selection:

haberdashery, ribbons, buttons, passementerie, 
threads, fabrics, felting, knitting, books, and so much more...

but, if you can see the square sign on the door...
pas de photos!

Entrée des Fournisseurs
8, rue des Francs-Bourgeois
75004, Paris