Most days it’s a fair bet that should the biscuit bug strike, I won’t have any buttermilk hanging around in the fridge (and if I do, it’s usually been hanging around if you follow me). Happily, you don’t really need store-bought buttermilk to make buttermilk biscuits. As I mentioned, there is no real buttermilk around anymore. What you find today in supermarket refrigerators is skim (or whole) milk that’s been spiked with lactic acid bacteria (so-called “cultured” buttermilk). That culture causes the proteins present in the milk to gel a bit, making it a little thicker than regular milk, and of course adds the acid, which is the really necessary bit.
Given that the thickness isn’t at all important for baking, and it’s really just plain milk anyway, there’s no reason you can’t just add a little acid to a cup of regular milk and be done with it. Truly. A tablespoon of vinegar will do the trick about perfectly. That or a tablespoon of fresh lemon juice or two teaspoons of cream of tartar. Let it sit for two minutes and bingo: instant home-made clabber. Of course you can also just swap out your buttermilk for an equal amount of plain yogurt since it’s pretty much the same thing, but it wouldn’t give you the same mad scientist buzz, now would it?