The problem with buying buttermilk at the grocery store is that the grocery stores don't always have it in stock. Another problem is that you may wind up with more buttermilk than you need and you will have to either freeze the leftover buttermilk or pour it down the drain. An alternative to either of these scenarios is to make your own buttermilk.
For 1 cup of buttermilk:
Measure 1 Tbsp of distilled white vinegar or lemon juice.
Pour the tablespoon of vinegar or lemon into a liquid measuring cup.
Add enough milk into the measuring cup to make 1 cup of buttermilk.
Stir. Let sit for 5 minutes and use in your recipe.