What should I use for old recipes that call for 'buttermilk'?
Given the variabilities in "buttermilk" from place to place and time to time, you should get sufficiently equivalent results by substituting modern cultured buttermilk. That's the job it was designed to do.

Recipes from the early 19th century and before are notoriously vague. They were generally written more as reminders of something you already knew through experience, rather than detailed instructions for creating it from scratch. Quantities and temperatures were much harder to control, and so recipes basically assumed you'd recognize a dough with enough liquid or a sufficiently-roasted quail. (One of my favorite instructions from an 18th century cookbook: "Cook until it is enough".)

Given those wide margins, you should find that simply using cultured buttermilk will make the recipe work. It's true that it will lack the tiny bits of butter present in "true" buttermilk, but there isn't enough to have a radical effect on the result. The really important parts (dairy protein, water, acidity) are present, in about the same quantities they would be in true buttermilk.

You'd certainly notice the difference if you were to just drink it straight. If that's what you want, you're just going to have to find somebody who makes it, or do it yourself. I've found it's often available at the kinds of farmers markets that insist on local producers. Personally, I find it nasty, but YMMV.

So if you're trying to revive an old recipe, just start with commercial buttermilk. You're going to have to tweak it from there anyway. If it needs richness, add butter, but probably only a teaspoon per cup.