Source  Good Life Eats

Recipe Added:  8/26/2012

Real buttermilk is best for flavor in baking, but in case it is not available, here are substitutions:

White Vinegar Add one Tbsp of white vinegar to a liquid measuring cup. Then, add milk until the amount reaches one cup. Let the mixture stand softened for 5-10 minutes. It should look curdled. Stir.

Cream of Tartar Measure 1 3/4 Tbsp cream of tartar + 1 cup milk. Whisk to combine and let stand softened for 5-10 minutes, until curdled and then stir.

Lemon Juice Add one Tbsp of lemon juice to a liquid measuring cup. Then add milk until the amount reaches one cup. Let the mixture stand softened for 5-10 minutes. The milk should look curdled. Stir.

Plain Yogurt 2 Tbsp milk + enough plain yogurt to equal one cup. Stir to combine.

Sour Cream Thin sour cream with a bit of milk until it reaches the consistency of buttermilk. Stir to combine.

1. http://www.goodlifeeats.com/2011/03/kitchen-tip-homemade-buttermilk-substitute.html