That's what I usually do too. I am unwilling to pay as much as they charge for cultured buttermilk. If I know I will need some, I might culture some milk with small dollop of sour cream or quark (the unheated kind).I'm with Katie, freeze it in recipe sized containers!
Also try making your own buttermilk substitute in the future.
Put 1 T white vinegar, apple cider vinegar or lemon juice in a glass measuring cup and fill to the 1 cup mark with milk. Wait 5 minutes and use in any recipe calling for a cup of buttermilk.
This is what I do because I never remember to purchase buttermilk.
My late father's favorite was Southern milkshake. Cornbread crumbled in Buttermilk. It's not bad if you like buttermilk.
I made a recipe with buttermilk last night and have quite a bit left. What are some good recipes to help me use that up?