Another method that's cheaper, less effort and you're more likely to have the ingredients in the fridge: add one tablespoon of lemon juice to one cup of milk and let it sit for 10 mins until it 'clabbers'.

Apparently you can use vinegar instead of lemon juice, but I've never tried that; I'm always making this to toss in a cake so lemon juice seems acceptable whereas the idea of vinegar is gah...

kodokan