There are many, many clothing stores where you can buy "western" clothing. The problem is not the clothes but the sizes. If you wear small or medium sizes, you will have no problem finding clothes. If, on the other hand, you are an XL-Long (like me) you will have trouble. Shoes, in particular, can be a problem. My US 13s cause looks of consternation with inn-keepers trying to find slippers for me. I can't imagine trying to find shoes for purchase.

If you are going to be in Kyoto, I know there is a GAP on Karasuma-dori a block or two north of Kyoto station - just south of Nishi-Honganji temple.

JR