Pretty much anywhere in the parks that sell postcards will sell postage stamps. The Emporium on Main St in the Disneyland park sells them, I have bought them there. Just ask for an international one. The postcards will take about a week to get to Australia though - they take longer than a normal letter for some reason, so if you don't want to arrive home before your postcards be sure to send them in time!
Hotels also sell them in their gift shops. Just don't get ripped off by buyingthem from thos machines that charge you more for the stamp than it's face value eg: to get a $1.25 stamp you have to put in $1.75, or even $2.