At MK, Toontown is a great place to find Princesses. There is a meet-and-greet room for the Princesses at the Hall fo Fame inside the County Bounty gift shop. Princesses tend to varry here, but Snow White, Cinderella, and Aurora can be found there most often. Belle appears at Fairytale Gardens where she holds a storytime session. Belle comes out, tells her story with the help of some children from the audience, and then does a meet-and-greet aterwards with the audience. Jasmine can usually be found in Adventureland near the Magic Carpets of Aladdin attraction. Ariel appears in her Grotto behind the Dumbo ride in Fantasyland.
At EPCOT, World Showcase is a great place to meet Princesses. Aurora and Belle are usually in France either in Belle's library across from the perfume shop or in the nearby garden. Jasmine is often in Morocco near the street. Snow White is commonly found between Germany and the Outpost, and Mulan is often in China around the entrance to Reflections of China.
At MGM, Princesses tend to be a little rarer, but you may find a couple near the entrance in the mid-afternoon.
AT AK, Pocahontas is pretty common at Rafiki's Planet Watch. You'll have to take the Wildlife Express train from Africa to get there.
Hope that helps.