These merchants can be found in all the towns and cities: Sanrio Harbour, Florapolis, London, Paris and Beijing. They can also be found in the train stations of Florapolis, London, Paris and Beijing. They sell some of the same goods at the stations like on the surface in the associated cities, but also sell some different clothes.
The things they sell change for each town. They not only sell clothes for the upper and lower body, but also glasses, shoes and most important: sewing guides for components and clothing to make your own equipment from materials. You will need to buy some of these guides to complete certain quests.