Lemon and Milk
Place 1 tablespoon freshly squeezed lemon juice in a liquid measuring cup. Add whole milk (low-fat versions are too thin) to reach the amount of buttermilk called for; stir and set aside for 5 minutes.
Yogurt and Milk
Whisk together 3 parts plain yogurt and 1 part milk for the desired amount of buttermilk. (For example, for 1 cup buttermilk, use ¾ cup yogurt and ¼ cup milk.)
Buttermilk isn't just for baking. For more inspiration, try these 18 recipes for buttermilk.