Shimmer, from my notes: A buttermilk Substitute: To each cup of milk add 1 Tbs lemon juiceor white distilled vinegar. Let stand a minute before using.
Unfortunately, I didn't note where I found this info and have never tried it. As a cup of milk will not break the bank, you can easily experiment. I've also seen vinegar suggested.
There's also Saco dried buttermilk (intended for use in cooking, not drinking). They have a website out there. Tony Cenicola had an article on it in the NYT. He says, Saco Cultured Buttermilk Blend can be found in supermarkets or directly from Saco Foods, (800) 373-7226. Apparently using the driec may require some adjustment of leavening agents. Check the Saco site.
Shimmer, I have tried the milk and lemon juice. The trick is to let it stand for 5-10 min. so that it thickens a bit.
My favorite substitution is non-fat or low-fat yogurt. Particularly in muffins, they come out so tender.