Just recently I wanted to make soda bread and it suddenly dawned on me that I had read a post on Clotilde Dusoulier's excellent food blog Chocolate and Zucchini, yonks ago, about a Breton product called lait ribot. I see it in the supermarket as a regular item, in the refrigerated section along with the fresh milk, and my memory of Clotilde's description was that if it wasn't buttermilk, it was a very reasonable substitute.
Using the other half of the lait ribot to make cornpone.It turns out that lait ribot is indeed a type of buttermilk. In the old days it was the traditional product, which was the creamy but low fat liquid left in the churn after you have made butter, deliberately fermented to increase its keeping qualities. (Lait is the French word for milk and ribot from the Breton word for a churn). These days the by-products of industrial scale butter making go into industrial scale 'not butter' spread making, along with various vegetable oils. Cultured buttermilk, or lait ribot, is now manufactured in its own right by adding a lactobacillus to milk to lightly ferment it, making it go slightly sour and curdled ie essentially, drinking yoghurt. Many French people would dry retch at the thought of drinking a glass of milk, but they adore a glass of lait ribot, especially if they are holidaying in Brittany.