Is there one?
How about some buttermilk? :)
I'd whisk a bit of water into regular yogurt to thin it out. Or use whey if you have it. It depends on what you're using it for.
Hi Crash. Trader Joe's has it. Regular yogurt would work too; you might stir in a little water to thin.
Kefir (am throwing it out as I've seen it suggested instead of buttermilk, but can't say I've tried it. Though might be even harder to find!).
Thanks, all! Found it at Trader Joe's.