When you first go to Ninn's Boutique in Numara, there is a person guarding the entrance to the back room. He will say it's off-limits to normal customers. You will need a password to get in. Leave the boutique and enter the Artists' Salon which is the building due south of the boutique on the same map. Talk to Hocke, who works at the boutique and gives you the password when he finds out you were not able to get into the back room. Return to Ninn's Boutique and you can get in to the back room.