These two above are right. Sometimes I also use half yogurt and half milk (for the texture of buttermilk) and some lemon juice. It's not quite the same as buttermilk, but using the yogurt gives it some of that tang that buttermilk has. I like this better than just using regular milk because it has a thicker texture.

Latoscana's got it right! When replacing buttermilk, just keep in mind that it has more acidity than other dairy products and you'll need to make up for it with vinegar (I like to use apple cider or white). Heavy cream is also going to, proportionally have much more fat in it - if you can, compare some labels and find something that is similar in fat content, and add a bit of acid!

A substitute for buttermilk is to add 1 tablespoon vinegar or lemon juice to a cup of regular milk.