Hey there.... well, there are a few places in the UK that sell gothic stuff that with a little work can be made to look like j-rock music. As for Japan, there's ID Japan I know who actually design clothes for various bands (you can buy the outfits but at IMMENSE cost!) and make there own more affordable gear. Their website is
http://www.id-japan.co.jp
I noticed you read a japanese so you'll be okay but the first time I looked at this site I couldn't read it and made my way around easily.. now I can (but no kanji!!) so its even easier.. I'm sure you'll be fine! There's also ba-tsu which Kyo from Dir en grey designed some clothes for and their web site is
http://www.ba-tsu.co.jp/
I hope some of this has helped! I dress visual kei as much as possible, but even living in the UK, its really hard to look more than just goth.