How to get something valuable back?

I left my son's Christening gown with my mother. I have asked her multiple times over the course of about eight months to send it back but she keeps breaking her word. I paid for it and it is my heirloom. She is in Florida and I am in Oregon. She is very stubborn about it and she's really frustrating me. I'm thinking of suing her to get it back but how do I do that when shes in another state? She really has left me no other choice.