How to make buttermilk
I just recently figured out that making my own buttermilk is dead easy. Take about half a cup of store-bought cultured buttermilk and pour it into a quart jar. Then fill the rest with milk (either fresh or reconstituted powdered milk will do), cover, and let it sit on the counter for 5–24 hours, until slightly thicker. I usually mix mine up in the evening and leave it overnight. (You will need less time in warmer weather, and more time when it's cooler.) It will coat the jar slightly when it's ready.
When you've got your buttermilk how you want it, keep it in the fridge. I have kept mine for up to two weeks with no problems.
Most of my batches have turned out similar in thickness to the storebought buttermilk, although one turned out as thick as yogurt (I probably left it out too long). All have been perfectly edible and work great in baking and salad dressings. Depending on the thickness and particular application, you can use it in place of milk, sour cream, or yogurt.
When you're down to about half a cup or so, just refill your jar with milk and leave it out again. You could keep the culture going forever!
Up until now, I've been using milk spiked with vinegar for a buttermilk substitute. But I've been trying to get more live foods into our diet, and buttermilk is just too fun and easy to propagate.
Next I will try starting a batch from powdered buttermilk (Saco brand), and I will see if that works. If it does, it means you could start your own live buttermilk entirely from pantry/food storage items. (Update: I tried it and not only did the reconstituted buttermilk smell positively foul upon mixing, it did not manage to culture my milk.)