I remember reading 2 years ago in the Globe that Kate's Butter of Old Orchard Beach (ME) was planning on rolling out real buttermilk in a Boston-area supermarket chain in the near future. I looked diligently for about a year at Whole Foods, Shaw's, S&S, Roche Bros, Johnnie's and Market Basket. Then I gave up the diligent search and searched occasionally.
And, lo, today at the Market Basket in Reading, there it was: Kate's "Real Buttermilk." No gums, no carrageenan, no vinegar. One ingredient: cultured buttermilk, made from the milk left after the butter is made. Now, it's still not pre-industrial old fashioned buttermilk - industrial centrifuges are too efficient at separating cream compared to old milkmaids (like my grandmother). But the product is not as gooey as the standard commercial product, and it leaves several tiny flecks of solids on the bottom of my Pyrex custard cup that I used to compare the buttermilks.
Oh, it comes in a lovely milk-bottle shaped plastic quart bottle and costs $2 ($1.99, to be precise if you dislike rounding).