Well, I assume that a lot of people doesn't know how to do that, because I didn't know until one the moms in Winnipeg Baby Wearers Group forum shared it. It's so simple yet satisfying!
All you need is whipping cream. 1 litre will give you about 1 lb of butter. A good mixer like KitchenAid would make your job a lot easier. First, whip the cream until stiff. Make sure it's cold to start with. Keep beating until the whole cream separated to liquid and butter. Strain the butter using cheesecloth. The liquid is the 'original' buttermilk. You can save it for drinking or baking. Rinse the butter with cold water and you can salt it to preserve, or you can use it right away if your recipe calls for unsalted butter.
Tip: For organic butter, you can buy organic whipping cream.