If you don't have buttermilk, you can use sour milk.
Make sour milk by adding a tablespoon of lemon (or vinegar) to enough milk to make 245 grams. Let sit for 5 minutes. That what I use for baking when I don't have buttermilk. Never used it for the marinade, but I think it would work.
(You can also use whey - but I don't think that's sold anywhere. I end up with whey when I make farm cheese or yogurt cheese, and there's only so much whey I can freeze for smoothies so I tried it with the chicken.)
