You can buy it through the Laughing Place Store. They probably get it from the Disney scrapbooking store in DTD, so if you are going to be there or know someone who is going they can probably pick it up for you. If not:
I fancied making my own confetti using a punch (I have the Mickey head and castle punches already) but I've still got to look into buying bio-degradeable paper to make it from, as I've been told that Disney only allow guests to throw bio-degradeable confetti or rose petals.