When I make pancakes and don't have buttermilk on hand, I frequently substitute in equal parts plain yogurt and skim milk. It works really well with pancakes; would this work with other (baked) recipes that call for buttermilk, such as buttermilk biscuits?
Can't find buttermilk? I would give it a try for biscuits, why not. When I read your subject line my first thought for a buttermilk substitute was plain yogurt. Good luck, let us know how it goes.
I suppose it would work fine in any recipe that calls for buttermilk. Real buttermilk is harder to find than real sour cream, so I generally use 1 tbs of (cider) vinegar to 250ml of (we use goats milk) milk. Let it sit for 10-15 minutes and it is ready to active my chemical leaveners.