does anyone know where i can change my characters hair after you leave mitchel's house?

Join the Kings and you'll have access to a fabelous hair dresser :hubbahubba:

who exactly?

http://fallout.wikia.com/wiki/Sergio.

i chose the NCR deal any other way to join them?

I'm afraid not, as you can only join by using your one favor you get from the King if you help him.

If you're on the PC, you can change your looks by opening the console and typing showbarbermenu or showracemenu.