Kelly,
There are a lot of options of what you can do.
First, you can visit a
Disney Store in your area, if there is in fact one nearby. I checked one out last weekend, and the selection of princess dresses is not always great, but typically they hit the newest (Merida from Brave right now), and the most popular (Cinderella, Belle, Sleeping Beauty, for example) pretty well.
Second, you could buy online from the Disney Store (same link as above). Your selection will be broader that way.
Third, you could wait until you get to Walt Disney World. Now you do not
have to go to Bibbidi Bobbidi Boutique to buy them, but the best selection will be at the World of Disney Store in Downtown Disney. It happens to have the original Bibbidi Bobbidi Boutique location inside it. But you are shopping for dresses there just as you would their shirts, toys, and household goods. No need to be a boutique customer to get the dresses.
With my girls, we prefer the third option. There are dresses to be purchased at stores in the parks, but we like to have that all sorted out before we get to the parks. We don't want shopping to get in the way of the magic!
Write me back if I can help further!
Have a magical vacation!