Where to buy large-sized shoes in Japan
If you are a foreign resident of Japan who needs to buy large sized shoes, try the shoe shop Kutsu no Hikari (靴のヒカリ) in Tokyo and Kawasaki. They stock not only extra-long but also extra wide shoes in many varieties, such as sandals, business-style shoes, hiking boots, and so on. The lengths and widths go up to well over thirty centimetres as well as extremely wide sizes.
Unfortunately their branch in Shinjuku 5-chome has now closed, but they still have branches in Kawasaki (a general shoe shop which stocks large shoes) and Okachimachi in Tokyo (specializing in large shoes only).
They don't have a web site in English, unfortunately, but their web site in Japanese is Kutsu No Hikari (in Japanese).
The shop is as seen on the right in Google Streetview. You can go to a map by clicking the link underneath the streetview.
This also claims to be part of "Kutsu no Hikari". The selection of shoes is very similar.