This one would be really hard to say without first knowing your tastes in music .. hm. I'd say the biggest name in JPop at the moment is still "Ayumi Hamasaki", though. XD Personally, I hate her, but a ton of people love her music - so you could check that out.

Some that I like are: (mixed JPop/Jrock) Utada Hikaru, L'Arc-En-Ciel, The Brilliant Green, Aikawa Nanase, Day After Tomorrow, Morning Musume - that's just naming a few. There's a ton of great stuff out there.

I usually get most of my older music off P2P sites. As for new music - there's one site I know of for the newest releases, but it's not really up for me to give to the masses XD But I'm sure you can find stuff out there. Even on anime.mircx.com, they have tons of soundtracks posted (Mostly by #Anime-Game-OST).
Soundtracks are a good place to start, too ^^ You can see who does what songs, and maybe you'll end up liking them.

I know ever since Naruto's second opening, I've checked out every release from Asian Kung-fu Generation.