You can approximate buttermilk by adding a few drops of vinegar to regular milk. Let it rest a couple minutes before adding to the recipe. The milk curdles a bit and creates the same effect as buttermilk. Since learning this, I’ve never bought buttermilk because I always have vinegar and milk on hand.
If that taste doesn’t suit you, be aware that they sell powdered buttermilk in the baking aisle, I think near the powdered milk. Much less perishable, of course, but still an extra expense if you don’t think you’d use it much.