Like many of the World Showcase pavillions, Japan only has one real merchandise location. It is a satellite of Japan's oldest department store, Mitsukoshi (the same company also runs the pavillion's full service food locations.) If memory serves me correctly, Kimonos and other apparell can be found in the back section of the store, between jewelry and snack foods. It's not that big of a store, and they have lots of neat items, so "having to wander around" will neither be that big of a task, nor will it be unenjoyable.
Have fun, and I hope you find one you like.