You can make buttermilk. All you need is raw milk, a jar and time.
Or you can make a buttermilk substitute that only takes a few minutes. Just add a tablespoon of vinegar or lemon juice to milk. Mix. Let sit for 10 minutes. How does it taste? No idea. No desire to taste it. Or you can make buttermilk that only takes one day and uses buttermilk with active cultures and milk. Mix one cup of cultured buttermilk with three cups of milk in a jar with a lid. Shake it up. Set it out of the way for 24 hours. Tilt the glass. The thickened buttermilk should coat your glass. How does it taste? It depends on the buttermilk you used as a starter. I’ve used Sky Top and Farm Friend, but I prefer the tangy Farm Friend. I found the Sky Top made a super thick buttermilk but tasted a bit sour. And it wasn’t a pleasant sour. YMMV.
My preference for buttermilk is to just use raw milk. I buy mine from Ty Llwyd, but it’s available from Mecox Dairy as well. You need to plan ahead because this takes several days. On the day you pick up your raw milk, measure out a cup and put it in a glass jar with a lid. Let it sit there until it clabbers. I totally forgot about mine and wasn’t quite sure how “clabber” looked. A few days later, I remembered the jar. It was clabbered all right, thick, kinda like yogurt. The directions I read always said to take a quarter cup of that clabbered milk and add another cup of milk to it. I didn’t want to waste those other three quarter cups so I just added more milk to the jar, shook it up and let it sit. It might have clabbered more quickly if I had just used the quarter cup. Since I wasn’t in a hurry, I didn’t care. You repeat this process of sub-culturing until the milk clabbers in 24 hours. Why would you want to do this? If you make it yourself, you know there’s no preservatives, no thickeners, no added salt, no added vitamins. It lasts for weeks. When you run low, you can use that last cup, add three cups of milk, shake it up, let it sit on the countertop for 24 hours until it clabbers, put it back in the fridge and you’re back in business. Best of all: the taste. So good I could see having a glass of it straight. It brought me to my knees.