What exactly are you looking for?
To be honest, most of the Japanese people here wear clothes from America. Brands like forever 21, Abercrombie, Gap are overwhelmingly popular.
If you're just looking for Japanese brands, and NOT what actual Japanese people are wearing, you could go for Uniqlo or Muji. Those are some cheap, mass-produced japanese clothing. Japanese clothing is terrible quality and awkward fitting. Unless you like skinny jeans and xxs t-shirts with the build quality of tissue paper, I suggest you seek out something else.
Oh! And I forgot about Nissen (if youre female) - I did an ad for them earlier this year. I'm the second guy to go over the ramp.
Nissen Commercial