Dahi is a natural unpasteurised yoghurt eaten as-is and an ingredient in India, aka curd in Indian English (different to curd in Br/Am E. though). Khatta dahi is more soured (khatta means sour).
What's a good commercially-available (outside of India/Asia) substitute? Natural yoghurt for dahi, plus some (how much?) soured cream for khatta dahi perhaps?
I don't think commercial natural yoghurt could be soured at home, since my understanding is that they pasteurise it (to prolong shelf-life) before re-introducing the desirable bacteria (but not the undesirables).
Or perhaps dahi is often only used because it's so common anyway, and actually if we're substituting it needn't be a yoghurt at all; we could just use (soured, for khatta) cream? - which is of course higher in fat, but so is buffalo milk, which is also used (I'm not sure whether it or cow's is more common) for dahi in India anyway.