What can I substitute for buttermilk?

There are several substitutions for buttermilk to supply the acidic effect of buttermilk. You could use 1 tablespoon of lemon juice to 1 cup of milk and let set for 10 minutes and stir. You could also use 1 tablespoon of white vinegar to 1 cup of milk and let set for 10 minutes and stir. You could also use 1/2 cup of sour cream or yogurt to 1/2 cup of milk. However, if you are making a buttermilk pie, I have found you must only use buttermilk.

One more to go along with what Marie said: 1.5 teaspoons of cream of tartar plus 1 cup of milk

Well, depending on what you are making, all of the above, and maybe consider creme fraiche if that is appropriate.

Thanks for all the helpful suggestions- I think I may try all of them and see which works the best for me.