Hi, Rachel! You really are a smart mom to think of purchasing your daughter's princess costume before you arrive at WDW. It actually does save you money by either buying online at the
Disney Store or by calling 1-800-328-0612. You can buy a princess costume from different retail stores or discount online costume companies. However, I've found that the quality is not as good as the ones purchased through Disney. The Disney Store usually has a promotion on shipping or merchandise discounts which makes purchasing outside the parks more affordable still.
Another little tidbit, you can have her costume delivered to your Disney resort and have them hold it for you until you arrive. Once you check in, ask the front desk cast members if they can have the costume either placed on her bed while you're away or brought to her to your resort room. Imagine her face when her royal gown is presented to her in such a grand fashion! (Just remember to let your resort know that you're having it shipped to them and to hold it until your arrival.)
Thanks for your question, and I hope everything works out beautifully for your little princess. Have a wonderful day and a magical vacation!