Churn your own butter. The leftover liquid is buttermilk. Modern buttermilk is a low fat milk that has been cultured with a bacteria to thicken and sour it a bit.
For cooking purposes, plain yogurt substitutes well or a cup of milk at room temp with a tablespoon (that much? I can't remember for sure) of lemon juice added. Let it stand a few minutes and it's ready.
Saco manufactures a powdered buttermilk I use in baking a fair bit. I've been pleased with it and you could probably order some. It keeps quite well unopened.
I've had a lot of luck with a cup of milk and (yeah, your right Phil) a TBS of lemon juice. It starts to thicken in a few minutes but left for hours or overnight in the fridge gets better results IMHO.
in order to make buttermilk with real milk, you'll need to add in some acid in there (i.e. real tiny lemon zest, tsp of lemon juice, etc). something that'll add acid inthere but won't change the flavor.