I never buy buttermilk, to me it is a waste as I don't use it often enough so it goes bad. Instead I have learned how to make my own and I prefer it. It is super simple to make, you probably have everything that you need already in your refrigerator door.
Now you won't have any excuses as to not make a dish just because you don't have any buttermilk on hand.
1 cup milk
2 tablespoons lemon juice*
Let sit for 5 minutes then use.
*You could substitute the lemon juice for white vinegar, but I prefer the lemon.