Sknuk just posted on Buttermilk/sourmilk and how to make it. I did not want to hijack the posting, but wanted to mention that you with that homemade buttermilk you can make creme fraiche and save your dollars for something else not so easy to make:
2 TB of buttermilk and a cup of whipping cream into a glass jar covered tightly. Shake or stir it well and let it sit at room temperature (not too too hot) for half the day - like you can use it that night if you make it in the morning or let it go overnight. You use it when it thickens. Stir it after the sitting and then pop it in the fridge.