Hi Christine,
Wow, that is a tough one as I have always had problems buying "Disney Dollars" while in Canada. A few years ago I asked at one of the Disney Stores here in Canada and their response was that they only sell them in the U.S. because of the exchange rates to other currencies always fluctuating! I have actually stopped buying Disney Dollars since then because whenever my daughter received them (and her favorite is Tinkerbell too!), she didn't want to spend them because they were too "special" and it became too expensive to buy them just as souvenirs.
If you want buy them ahead of time, you can try the Disney Merchandise Department at 407-363-6200 or
wdw.mail.order@disneyworld.com. I am not sure if they can send you Disney Dollars or not, but they seem to be able to make all kinds of magic happen with everything else that you may require, so I can't see why Disney Dollars would be any different (they of course will charge you in U.S. funds though). If all else fails you can try EBay, but I am not sure if this would be worthwhile budget wise and also for authenticity purposes.
By the way, don't forget to stop at any of the Guest Services locations in any of the parks and ask for a complimentary "Birthday" button for your daughter to wear during your trip for some possible "extra magic" and attention she may receive from cast members throughout WDW. What a great surprise for her birthday......a trip to Walt Disney World, Wow! I hope you all have a magical stay.
Best wishes!