You need to make a little butter!

If you have no buttermilk available at all, then you will have to make a small amount to start with. I haven't actually done this except with a churn, but my youngest brother made some in home ec years ago. Put heavy cream into a jar (plastic would be safer, they used a Tupperware container, but it still works) and tightly seal it. Now, roll it around. The kids just roll it back and forth to each other. After about 20 minutes or so, you will have butter. The liquid remaining is buttermilk.

There might be better instructions for making butter here on the web or on kitchenlink, but I think this is as simple as it gets. I believe that a hand mixer does not work as it churns too fast. A stand mixer goes slow enough. If you can't get cream, well, I'm out of ideas then. :)