Periodically I have a conversation with someone - usually American - about what one can use as a substitute for buttermilk. Generally we agree that we have never, or very rarely, seen buttermilk available on the supermarket shelves, yet it is an ingredient that certain recipes traditionally contain and our mothers or grandmothers seem to have used reasonably regularly. We agree that the easiest substitute is natural yoghurt thinned with milk, but we occasionally use milk that has gone sour or faiselle (like cottage cheese) that is past its best, just like our mothers and grandmothers would have. We suppose that the other possibility is using drinking yoghurt, but since none of us ever buy the stuff, we've never tried it. I've also used the whey saved from mozzarella cheese as a substitute for buttermilk in cakes.
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.