It is in Fantasyland attached to Cinderella's castle. From the Disney site:
Bibbidi Bobbidi Boutique in Disneyland Park is a unique beauty salon where little girls are transformed from head to toe into little princesses. Choose from a variety of special packages that include everything from hair and make-up to princess gowns and accessories.
For reservations, please call (714) 781-STYLE or (714) 781-7895. Guests under age 18 must have parent or guardian permission to call.