For more widespread artist albums I use Demonoid.me. Alternatively, for everything else, I use RuTracker.org. Both of these compliment each other very well and will never cease to please you and your music collection. I must add that if you do indeed torrent that whenever you can you support the artists by buying albums, merchandise or concert tickets. I know how difficult it can be at times to want to listen to everything but not have the disposable income to buy it all.

Isohunt + µtorrent?

best-selling albums are getting harder and harder to find

there's an abundance of metal albums, though

Mediafire's dead IMO. If anybody's actually finding albums on there, I envy you.

I use Soulseek now. It's slower than Mediafire (90KB max so far compared to 1MB max), but I'm actually finding what I want easily.