You have alot of choices to see some princesses! To see the most guarenteed, one of the character meals would definately do (Cinderella's Royal Table at Magic Kingdom, or Princess Storybook at Epcot) but they are hard to book and sometimes fill instantly 90 days out.
In the parks, there is a room in ToonTown in the ToonTown Hall of Fame building that usually has Cinderella and maybe another princess. Also, many times we seen three princesses at once by the Big Hat at MGM (but I read that some of these characters may be by the entrance nowadays instead, so I am not sure where to look.) The princesses we seen include Cinderella, Jasmine, Mary Poppins, Pocahontas, Mulan, Megara, Snow White, and Belle (though not all on the same day!)
I also seen princesses right after you enter Magic Kingdom, usually near Exposition Hall. We seen Mary Poppins and Aurora there.
Every day, though, the character lineup can change, so you can never be 100% sure who will be there!