- Youtube (or Google Video Search for a wider spread) - many Japanese bands have common English words as names and so are hard to find (like "Straightener" or "Half-Life"). Add "PV" to your search and you will find their music videos (PV is an almost exclusively Japanese term, meaning "promotional video").
- Discovering Indies - Again on youtube, I recommend a related videos chain - where you just keep clicking on music videos of related artists. You'll find some gems then! I find the sources of most of my videos tend to be Victor Music, UK Project official, and Avex Network.
- Download sites - There are several live journal communities, such as Just Download, or torrent sites like JPopsuki, but I don't use those much (my school network doesn't support torrents). What I do is just Google the band (and maybe album name) + megaupload, rapidshare, or mediafire. Even with the rarest and newest material, works like a charm. Though of course only download what you already have, delete it in 24 hours, etc yadda yadda. Luckily a lot of friends randomly send me music too to keep up my library.
- Japanese music charts - In Japan, the music chart is called Oricon, and every week they release a top-30 list that serves as a good starting point for studying Japanese music. Currently I can't find a good source, but the best shows to watch are "Music Station" and "CDTV" (Countdown TV).
Hope this helps you guys on your quests to find some awesome music! ;-)