I had a look at buying points from the US when the AU$ was strong, but I couldn't find any confirmation that the points would work on Australian systems. Now, with the crumbling exchange rate, we don't fair quite so bad on a world wide scale.
If you do find something better, I would love to know as I am sort of burning through points at the moment (GHWT, SSFIITHDR and so on)....
AFAIK, buying points online may only work for US accounts if they are "US-based" points. Won't work if you have an Aussie XBox Live account.