Buttermilk is an ingredient in many of the Ozark/Southern baking recipes that I grew up with. Unfortunately, I do not normally buy buttermilk, but I do buy plain yogurt by the quart! Yogurt makes an excellent substitute for buttermilk.
Many websites say to substitute 1 cup of yogurt for 1 cup of buttermilk, or to use 3/4 cup of yogurt and 1/4 cup of milk. In my experience, this is incorrect. The consistency of yogurt is much thicker than buttermilk and does not contain enough liquid for baked goods.
My kitchen-tested substitution is :
1/4 cup - 1/3 cup yogurt blended with water to measure one cup. This mixture will give you the same liquid content as store bought buttermilk.
Baked goods will have a tender crumb, not crumbly, and have a slight, tangy taste. Cornbread made with this substitute will not be crumbly, or dry when you reheat it the day after it is made.