A good substitute for buttermilk is acidified milk. For each cup of buttermilk in a recipe, add 1 tablespoon of vinegar or lemon juice to 1 cup of milk. Stir the mixture, and let it sit for five minutes.
Alternatively, add 1 3/4 teaspoons of cream of tartar to 1 cup of milk, or use unsweetened, unflavored kefir in place of an equal amount of buttermilk. Thin 1 cup of plain, unsweetened yogurt to the consistency of buttermilk by whisking it with about 1/4 cup of water. Equal parts sour cream and water whisked together can also act as a substitute for buttermilk.