You can get them in the Magic Kingdom, I always got to The Chapeau on Main Street, but I am sure that you can get them other places. You can have them personalized there. I am not sure if you can leave them there all day while you are enjoying the park, but you can definately come back and get them in 30 minutes or an hour, in fact, sometimes the wait is that long to have them personalized, or have them sent to your room if you are staying on property. They don't have to be ordered online in advance unless you want to bring them with you. Hope this helps!!!
They are about the same price, maybe exactly the same price (only if you get them at Disney you don't pay shipping). Ears aren't super expensive unless you build your own and get one of everything for them. Hope this helps!!!