If you want info, just google the band. google.co.jp would probably work better for japanese bands. Some bands have sample music/videos in their sites. I also find youtube a great way to check out a band. Search in english, and japanese. If you like vk, there's lots of sites dedicated to it. If you want some underground bands, audioleaf is a great japanese site.
for english info, just look in any j-music related forum. wikipedia or wiki.theppn has info on lots of japanese artists. Want to know some lesser known mostly non-vk bands, check out IiNihon(link in a sticky in this forum). Though it's not updated regularly >_>
Jpopsuki is the best tracker there is for Japanese music. If they don't have it, search on some P2P program. If that doesn't work, just ask around. I recommend soulseek as a good p2p where you can search for music, and ask people in a chatroom.
J-rock is such a broad catagory that I can't really give you any details. If you really want to find different bands to listen to, you can't really just look in a couple of places.
The best resource though, is actual people, so if someone mentions a band you'd be interested in, but can't find any info yourself, ask about it.