Ask your pediatrician for a referral.
Well, you certainly might want to find a children's psychiatrist. If you google that you might find someone in your area. But, I think I would first talk to your school district. They have school psychologists who should be able to help you for free. If your own elementary school is not helpful, call up the district's special education section and see if they can help.
What makes you think she/he has mpd?
Annie is right. That should be your first step.
Most respected psychiatrists don't believe that multiple personality disorder even exists. This was a faddish diagnosis in which a doctor or doctors actually convinced the patient that he or she had several different personalities as a mechanism for dealing with extreme stress. 'The Three Faces of Eve' idea makes for good reading but that's about it.
This is not to say that the patient does not have severe psychiatric issues in need of intervention. It's just not multiple personality disorder.
Ya, that was in the back of my mind when I asked her what made her think it might be mpd. Thank you for your input.