Avant Garde Project brings many great 20th Century Avant Garde Music that are out of print in flac format for followers of this great "genre" which is much too creative to be categorized into a genre. Direct downloads as well as torrent downloads are possible. For beginners there is a good section of editors picks. For opening flac (and almost all the audio) files I recommend Media Monkey.
There is a also a large collection of Turkish Electronic Avant Garde Music Pioneer Ilhan Mimaroglu.