I needed help also on this one , see if you like this, worked for me ..:)

Ingredients
3/4 cup regular milk
2 tsp vinegar or lemon juice

Preparation
Add 2 tsp vinegar or lemon juice to regular milk, let stand a few minutes to sour, instead of buttermilk.
Comments
This came from a friend's banana bread recipe that calls for buttermilk. Works great!
enjoy ~
Rosalie