I purchased mine in advance through UndercoverTourist.com. I actually went the the Mousesavers.com website, subscribe to their e-newsletter and click on the link in the newsletter (comes out every month) for the best price. I had zero issues with them, they ship in to Canada free and when we got to our Disney resort had no trouble having them add them to our room keys (which also doubles as your park "key"). Even got to keep the originals as souvenirs!
The advantage to buying them early is to spread the cost of your trip out and avoid the yearly ticket price increase that usually comes in August but can happen at other times of the year. You don't have to buy them all at once from
UT either....buy one at a time if you need to!