Some of my favs are
Asian Kung fu generation
Uverworld
aqua timez
do as infinity
l'arc en ciel
hyde
high and mighty color
kelun
etc...
Basically what I do to find more japanese music is, if I watch an anime, listen to the opening and closings, if one interests me I youtube the band and listen to parts of the other songs. I just download a sample ablum them to see how they are. then branch off from there. Also youtube has "similar to...blahblahbla" so thats helpful too. Good luck ^^