I get mine when the
Disney Store or Disney Shopping.com has them on clearance. It just depends on which styles you like. Right now, the park ones are the most twirly. Depending on the "preferences of your personal princess" (say that 3 times fast), this could be good or bad.
The ones at DL are considered the "Deluxe or Premium" style and are priced accordingly. Also, they change up the design several times a year. Right now the current Disney Store version is a "ballerina style" with a shorter skirt. They may still have another, more traditional version, I think.
You know what I honestly would do if you still have some time... I would first search eBay and the Disney shopping websites and see what style you guys like. That may tell you where to buy. You can often find used ones that are in great condition. Just make sure you get the shoes ahead of time - like I mentioned above - to break them in. When the dress style is changed, so is the shoe and accessory style. The old ones move into clearance....
(Can you tell I used to be a Disney Store CM once upon my life? I know my princess stuff.....)