I did see online that you can freeze buttermilk. Use an ice cube tray to freeze 2 tablespoon portions, then throw them into a freezer bag. This is great, because I rarely use the whole carton when I do get one, and then the rest doesn't go to waste, and I don't have to buy a whole new carton for another recipe.
The same article also had two loaf recipes that call for buttermilk... I don't need lots of baking around the house, so I'm looking for opportunities to try both of them, too!