Here are just a few useful links for your Japanese music research, in no specific order:
Japan Metal Indies
-A small but detailed site with precious information on several classic Japanese metal bands, as well as more personal details about the site owner’s collection and other Japanese metal related goodies.
vk.gy
-An extremely comprehensive visual kei database. The amount of research and detail on obscure bands is astounding, definitely a great resource.
The Metal Archives
-The largest single-genre focused music database online, there’s over 1,500 Japanese bands archived across numerous sub-genres, many in significant detail, another extremely useful resource.
Beat of Japanese Metal Motion
-No longer updated, but a valuable resource worth exploring in the Wayback Machine.
VGMdb
-A heavily maintained site covering soundtracks across countless video game IPs. Not exclusively Japanese, but a huge portion of the site inherently is.
Japanese Metal Forum
-This, as well as at The Metal Archives, is probably the best spot where I can personally help you if you need to research a band. Other regular users put a lot of effort into digging up info here too, if you’re in need of assistance in researching a particular group or musician.