Where can I find buttermilk in France?
You'll want to look for one of two products either lait fermenté or lait ribot.

Lait fermenté is a cultured buttermilk. This is homogenized and pasteurized milk which has been cultured with lactic acid bacteria. This is the de facto standard buttermilk in modern times. If you go to an American grocery store and buy buttermilk you are buying cultured buttermilk.

Lait ribot is traditional buttermilk. It is the fermented liquid leftover from churning butter. This is not commonly found in the US and other western countries.

See also: Buttermilk