There is a Head Porter shop in Harajuku on
Takeshita Dori, but I don't think they sell clothing, only bags, wallets, backpacks, iPod covers and things like that.
I have never been to the Porter shop in Osaka, but maybe someone here has and they can confirm whether or not they carry clothing as well as bags? Also, as far as the website, it seems like some of the most exclusive items are available only at the shop - not on the website, although some resellers might offer them for a markup.
Maybe your best chance is Rakuten or Yahoo Japan Auctions?