Most people view CDs and MP3s as opposite ends of the music acquisition spectrum. Death metal, being an underground genre in which musicians perform for their art and not primarily for profit (although most hope to profit so they can avoid those insidious soul-killers known as day jobs), encourages "tape trading" or copying of albums and has extended that to MP3 trading. The theory is that people who like what they download will buy it, but if it's garbage, they'll never consider buying it; this way the "try before you buy" school of MP3 usage becomes a kind of natural selection. Much of the classic death metal is no longer in print and MUST be traded as it cannot be purchased.
Places to Get Death Metal MP3s
Audiofile
the Dark Legions Archive MP3 trading board.
Neoclassical Hub
requires DC++ software; tight-knit community trading most metal, electronica and classical MP3s.
USENET
you might as well use these gigabytes of traffic otherwise dedicated to warez, porn and Viagra.