If you are ordering under $50, WDW can do this over the phone, but this is their max to charge to a credit card. Call the 800 number and listen for the option that gets you to merchandising. This is the info I received from them. If you need more, you have to send a money order in US funds with an extra $10 for shipping and specify your denominations (1,5,10) to:
WDW Ticketing Fulfillment
Attn: Disney Dollars
PO Box 10140
Lake Buena Vista, FL 32830
I did this before Christmas and with snail mail it worked out to 2 weeks turn around - pretty much all mail being the delay, judging by the postmark. There isn't any other way to get DD in Canada.