The key really is the buttermilk. You can use substitutions, and they will still be good, but not the same.
If you want to know how to keep fresh buttermilk on hand to use all the time, follow these steps. Pour 1 cup cultured buttermilk in the bottom of a quart size canning jar. Fill with milk, put the lid on and shake. Leave out in a warm spot for 12 - 24 hours until cultured. (Yes, it is safe!) Put in fridge. Repeat process every time you get low or need fresh buttermilk!