With regard to bitstream formats, MP3 files are easely found on the web so I won't give any pointers. Vorbis.org is the homepage of the new emerging star Ogg Vorbis and provides a small selection of files as well as links to other files.

Soundtrack files now. Many can be found at:

• Return to Stage 9, an online music disk with very good stuff available in either the original format of in the MP3 version. Most of the files on the site use at least 32 channels so if you don't have a StrongARM powered machine forget it. Even for SA owners I recommand playing "The journey" without volume ramping and at only 31KHz because 64 channels of pure 16-bit samples seems too much for the slow RiscPC bus (and I havn't yet implemented the resonant filters some of the file uses).
• ModArchive , a lot of files can be found on this site but I have not explored the site too much.
• Scene.org.
• United Trackers.
• The Hornet archive.

Try composers like:

• Basehead, Hunz, Necros (now known as The Apha Conspiracy) and Jeroen Tell (WAVE) from Five Musicians.
• Purple Motion and Skaven from good old Future Crew.
• Andreas Vickund, Bert, Chuck Biscuits, Oona and Wayfinder from Kosmic.
• Corpse and Sander of Hell Riser Group.
• Chris Jarvis, Garfield of RD, Leviathan, Melcom, Mick Rippon (MickRip), Nightbeat, Nolan, Stinger, Xerxes.