Most J-pop/rock is easy to find on the popular peer to peers (kaaza and winmx), but if you're looking for punk, your best bet is soul seek.

I've been hooked on J-pop since the very first theme song I heard in an anime. I cant for the life of me remember which one, though I do remember rewinding the tape of SM episodes to listen to Moonlight Densetsu 50 thousand times.