Between all the parks and downtown disney, they have pretty much every princess thing you can think up! One of the best places for princess stuff is in the tent where the princesses are in toontoon. That store will put you on princess overload.
Surprisingly, my 3 year old went crazy over the Princess jewelry instead of the big stuff. She wanted a princess charm bracelet and a cinderella necklace. I think those were around $10.00 each. She also loves Aladdin, and she really wanted a Jasmine costume. The costumes are expensive at WDW. They are around $70, and that doesn't count the crown, shoes, wig, and any jewelry you want to add. I don't even remember how much it was when they totaled it up. I would recommend buying some cheaper costumes at home before you go. DD has been twice (once at 2 and once at 3) and she has never asked for costumes of a character if she already has one at home even if the WDW version is very different.