Cheesecloth

No idea where you'd find it, but the word in french is Etamine which may help you to find it. https://fr.wikipedia.org/wiki/%C3%89tamine_(tissu)

Sorry, that link didn't work properly, try here:
http://bit.ly/2mnXwvm

If you can't find any locally, you can get it delivered from Amazon France:

https://www.amazon.fr/s/ref=nb_sb_noss?__mk_fr_FR=%C3%85M%C3%85%C5%BD%C3...

By the way, if you need it for filtering cold liquids you can use J cloths from the supermarket, which work just as well.

I buy it at the supermarket. étamine in French, kaasdoek in Dutch.
Commonly used, commonly found, supermarket, kitchen store, hardware store. Dille et Kamille sells it.

Hi you can buy small squares of it in delhaize in the cooking isles