In “Where can I find buttermilk thread” from last year, the only solution was Dragonbones’ suggestion to get the culture and make your own. Since I am not a wizard with fantastic magical powers, that’s not really an option for me. But today I made pancakes using the ‘AB Yogurt’ (sugarless) instead of buttermilk, and it worked fine.
Actually, the stuff tastes a lot more like buttermilk than yogurt anyway. Next project: AB Yogurt (sugarless) biscuits.
Unsweetened yogurt and sour cream are common buttermilk substitutes, yes. Sweetened yogurt (or even flavored) is ok if it’s going into something sweet (or compatibly flavored), of course-- just reduce the remaining sugars accordingly.
Another substitute (per cup fresh bm.) is 1/4 c buttermilk powder + 1 c water. Order a bag of the powder next time you’re home, or pay for shipping, then keep that refrigerated or frozen.
Other substitutes use milk plus a small amount of something slightly acidic, 1.5-1.75 t cream of tartar /c. (for baking).or 1 T white vinegar or lemon juice / c. milk.
The culture is easy to get, btw, and light so shipping costs are low. After you make a batch of bm., pour it into ice cube trays, freeze it, put it in freezer ziplock bags, and pull out and thaw when needed.
For a cup of buttermilk, I’ve always put 2 tbsp lemon juice in a measuring cup, then topped up with milk, and waited 20 minutes or so. As far as I can tell it’s always worked, but never having used real buttermilk, I suppose I can’t be too sure.
That’s the standard US cookbook substitution directions for buttermilk, though most I know of tell you to wait 5 minutes.
http://frugalliving.about.com/od/condimentsandspices/r/Buttermilk_Sub.htm
However, I second the Unsweetemned AB yogurt drink suggestion. I drink both of these things all the time (AB in Taiwan, lowfat buttermilk in Seattle) and the taste is very nearly identical.
If you want to know WTF buttermilk is (it’s not high-fat regular milk or butter in another form…) you can read this: http://homecooking.about.com/od/cookingfaqs/f/faqbuttermilk.htm.
I am so crazy about the AB unsweetened that a few weeks ago when I left Taiwan, I put 6 big bottles of it in sealed bags in my suitcase. Everything survived. Looking at them side by side, 100 grams of AB yogurt has 0.2 g more carbohydrate but otherwise, they are nearly identical nutritionally… 56 calories for buttermilk and 57 for AB unsweetened. That’s within the range of a rounding error. 
The one caveat to all of this is that most buttermilk in the US will be full-fat and therefore will have higher calories than the AB yogurt. I don’t think that’s going to matter for most recipes though, since baked goods are going to have a fat source other than the liquid, typically.
You can get something similar here, mix a sachet with 1l of milk, leave it out for 1-1½ days this time of the year, less in the summer and you got something very similar to buttermilk afaik.
The packs look like this and it costs about NT$250 and contains 10 sachets.
Yeah, I saw that brand (unrefrigerated) at the Minquan bridge DIY shop, and I’m told health food stores carry it too.
Make your own buttermilk? What do I look like, Gandalf?
With a beard, you might, yes. Better start working on that, eh? 
Take heavy cream, add some yogurt starter culture, when sour and partly set start shaking until butter forms … the remaining liquid is the ‘buttermilk’ that can be improved texture wise by adding some milk … really, that’s how they made buttermilk ages ago, but the cream was left for a week or so in a cool place (souring process) until enough to make butter …
That’s not the kind of butter milk that’s being used today though, but I guess it works if you really want to make an effort…
The stuff I make takes no effort at all and you can make it straight in the milk bottle/carton…