[Trillium]

Adding a drop of vinegar, if you don't have lemon, to milk will also sour it to give the buttermilk taste  

I used to do the vinegar but found the taste was harsher than lemon juice, which is what I always use now. I keep a bottle of ready lemon juice in the fridge at all times. Much cheaper and handier than real lemons and in baking it's hard to tell the difference. Just allow a few extra minutes for the souring.