John,I don't think the buttermilk sold in stores today is really quite the same as the oldfashioned buttermilk left over from making butter. Now it's like a cultured product, thicker than regular milk. You can substitute regular milk for buttermilk by using 1 Tablespoon vinegar or lemon juice and adding regular milk to make to 1 cup of buttermilk. Let it sit for a few (5) minutes for the chemistry to work.
My Dad used to make a cultured drink called viili (I think that's the spelling--I believe it's originally from Finland). It is the consistency of runny yogurt but more tart, and I don't care for it by itself, but it works well in the place of buttermilk or sour cream in cooking. We always made it with nonfat dry milk. You can buy the starter through the mail.
Maggie