Hi Sara,
Since this can change daily, your best bet is to pick up a Times Guide when you enter the parks. Character Meet & Greet times and locations are listed on there. You can also stop by Guest Relations and ask them to check the character hotline if there is a specific one you are looking for that isn't listed on the Times Guide.
Princess Tiana can be found in the Magic Kingdom in Liberty Square behind Ye Olde Christmas Shoppe.
Ariel has been seen in Adventureland lately and Snow White has been in the Germany Pavilion in Epcot. Again, this can change daily though, so be sure to check when you arrive.
Also, if you are interested,
Rapunzel is in the Magic Kingdom's Fairytale Garden.
Have a wonderful vacation,
Kaylene