Can highly recommend Vintage Sports Shibuya. It’s a nightmare to find, but google maps takes you broadly there, then you go into the building and up the stairs, and into a room full of football shirts for sale. Visited there in January and came away with a Japan Shirt and a Kanu Portsmouth shirt, both of which are good value. They had a decent stack of Arsenal kits as well

I was there last month and randomly came across a store called Kamo located in Shibuya, they have some cool memorabilia in the store. I unexpectedly ended up buying a few of jerseys and the employees were extremely knowledgeable when it came to the kit numbers and patches.