In my continuing efforts to bend milk to my will, I made buttermilk over the weekend using bacteria from Cultures for Health. Cultured buttermilk is about cultivating bacteria. You'll need to make a fresh batch every week with some of the old batch, though. You can find the directions on Cultures for Health's website, or probably at any number of stores, but that's the one I used.
I like this solution because I end up wasting a ton of buttermilk. I need it rarely, and the stores around here sell it only in huge cartons. If you're not into making buttermilk but have the same problem, I found the powdered buttermilk works pretty well, too, and it lasts forever.
The instructions don't explain that the buttermilk may take longer than the recommended 12-18 hours to set. Mine took about 25 hours, so hang in there.
Note for bacteria geeks: There seems to be some debate about Streptococcus lactis's name. I'm going with the name Cultures for Health used.