They usually have it in the dairy section here in Australia. Woolworths have it on their online ordering site, as do coles if that helps.

But if you can't find it you can make your own. There is a recipe here:
http://www.thekitchn.com/how-to-make-a-quick-easy-buttermilk-substitute-cooking-lessons-from-the-kitchn-185757

But you can also do it by making your own butter from cream:
http://www.wikihow.com/Make-Butter-With-a-Food-Processor-in-Two-Minutes

You just squeeze the buttermilk out as you make the butter!