Unlike Reyes which retail well over 200 dollars in the U.K. and below 100 in Mexico there is no such difference in case of Winning. Winning, again unlike Reyes, are very expensive to make, so the price is always high and you are not going to get a "deal" like 20-30% off unless there is some defect (rarely appears).
Moreover America-Ya has some of the cheapest prices in Japan, so locally you can easily end paying more for the gear itself than Mitani charges for the gear, shipping and handling.
Also, with all respect, Japan isn't an easy country for a foreigner. Even if you get some address it might take you several hours to get there and find it. All in order to save (and that if you are lucky) like 10-20 dollars. Does it worth you?
So I would advise, if you accidentally reach some boxing gear store, go inside and check the stuff, but keep in mind America-Ya prices. Other than that it is a waste of time.
Also, I would like to remind that we do not buy Winning gear from rakuten, we buy it from America-Ya store. There are thousands of independant stores on rakuten, America-Ya is just one of them. Also rakuten itself (unlike amazon) doesn't stock anything.