Buttermilk substitute?
There are a few possibilities:


1 cup milk plus 1 Tbsp lemon juice or white vinegar, let stand for 10 minutes
1 cup milk plus 2 tsp cream of tartar, let stand for 10 minutes
2 parts plain yogurt to 1 part milk
Plain, low-fat yogurt
Sour cream
Molasses (if batter requires baking soda)


I have used the first two with success.

Source: Cook's Thesaurus