The Hall of Fame at MK always has at least 3 Princesses in the Princess room- usually Snow White, Cinderella, and Aurora. Others show up either randomly or when one of the three regulars can't make it.
World Showcase is another great spot for Princesses. Belle and Aurora can usually be found in France, Snow White in Germany, Jasmine in Morocco, and Mulan at China.
A couple of other good locations to check out are Ariel's Grotto in Fantasyland (it's the only place outside of character meals where Ariel appears), near the Magic Carpets in Adventureland (Jasmine often shows up here), Fairytale Gardens just to the right of the Castle (Belle has a storytime and meet-and-greet here a few times every day), and Rafiki's Planet Watch at AK (usually your best bet at finding Pocahontas).