As others have said, I'd recommend getting your eyes tested at an Optometrist (bulk billed), then asking for the prescription. They'll be grumpy but they have to give it to you so don't leave till you have it.
Personally, I've been buying my glasses online through this mob:SelectSpecs
They're a UK business, with REALLY cheap glasses, starting at around $14!
I've bought a couple of glasses and a pair of prescription sunglasses.
The only issue is that you can't try them on, obviously, so that's a bit of a gamble, but you could always try a few on in a store here and pick something similar.
I've had great service through them - the glasses reach you in about 2-3 weeks from order.