The Avant Garde Project has a large collection of free music that you can download, including digitized LPs of long out-of-print electronic music works:
The Avant Garde Project is a series of recordings of 20th-century classical-experimental-electroacoustic music digitized from LPs whose music has in most cases never been released on CD, and so is effectively inaccessible [...]

 

 A Fluttering of Wings [14:47m]: Play Now | Play in Popup | Download

Electro-acoustic music is a term used to describe a broad range of modern classical electronic music. It often explores the interaction of natural and electronically generated sounds and effects.
The term electro-acoustic refers to a process that happens in any microphone or loudspeaker - sound is transformed into electrical signals, and then transformed from electrical form [...]

Morton Subotnick is one of the innovators of classical electronic music. His early work with Buchla synthesizers took him to the peak of the avant garde electronica world of the 60’s, while his later works have explored the ways in which live performers can interact with electronics.
His site provides a history of his composition, [...]

Morton Subotnick is one of the great living composers. He has been heavily involved in electronic music since the 1960’s, and created some works which helped define the world of academic electronica. He also has been an innovator in works that involve eletronics and acoustic instruments, acoustic instruments manipulated electronicly, and other multimedia combinations. His [...]