I don't know anyone that regularly keeps buttermilk in their fridge. I often will decide to bake and realize that buttermilk is the one ingredient I am missing....frustrating. There is a quick cheater method to making buttermilk that will get you by:
1 Tablespoon vinegar or lemon juice + 1 cup of milk
Let the milk curdle for about 10 minutes and use as your recipe specifies.
I still do buy buttermilk if I know in advance what I am going to be making. It may all be in my imagination but I feel like the recipe turns out better with actual buttermilk.
I made the above muffins for Gabby's 2nd birthday. I remembered the buttermilk. I'll post this recipe and details of the party later in the week.