Yogurt can serve as a volume by volume substitute for buttermilk in recipes. A mixture of 1 cup of milk and 1 tablespoon of vinegar is another substitute for 1 cup of buttermilk, as is a mixture of 1 cup of milk with 1 3/4 teaspoon of cream of tartar.
Use heavy cream to make your own homemade buttermilk. Pour the heavy cream into a canning jar and shake it until it turns into whipped cream. Continue shaking the whipped cream until the milk particles separate from the butter particles, forming a yellow glob in the jar. Pour out the liquid part and use it as buttermilk.