I just came across this site featuring C64 and Amiga music for sale.

http://www.c64audio.com/

The Amiga section can be found here.

They have the three Immortal remix albums for sale, plus another called Mechanical Dreams which sounded good from the part I listened to (you can listen to some of the music streaming from the site). These cost from £4.99 to £15.99.

The price of C64 tracks is also not that expensive either at about 80p each.

I will add this site into the main weblinks on classicamiga.