Musée de l'Art Moderne de la Ville de Paris! What a mouthful, right? It was free as it's being renovated and only part of the permanent collection is visible. That said, we saw work that we hadn't seen there before, so it was a treat.
There were many dealers in paper goods, magazines, labels, receipts, magazines, and books, really old books.
Mostly things were too expensive for us but we found a small notebook that was old but only 4 euro. And we got two almanacs from the 1920s for 5 euro each. I got some old stamps for 1 euro. It was fun.