Buttermilk is pretty unrivaled when it comes to biscuit-eering, so let's go ahead and just admit and accept that you can't really replace it. You can't substitute anything else. You need its acidity to help the baking powder do its thing. You need its tang to give flavor and depth to the biscuits. You need its inherent creaminess to lend tenderness to the finished crumb. All this said, if you forget to buy buttermilk - or can't find it - you can make it yourself by simply combing 1 cup of milk with 1 tablespoon of lemon juice or vinegar. Let this acidified milk mixture sit in the fridge for at least 30 minutes before using, and it will act just like buttermilk.