I'm not so sure about AUS, but the best bet in the states is still garage sales or flea markets or the like. eBay has everyone looking for the same thing, and for the most part.. knowing what it is they want.
If you go to a garage sale, you'll find parents selling off their kids old NES games for $1 a pop. And the good thing is, a great number of "good" games on the NES were so popular, almost everyone had them.