They are free and can be downloaded to the computer, burned to a disc or loaded to an mp3 player. They are the books in the public domain – I have found many, many interesting books for history and literature read-alouds here.
http://www.libraryanded.com has some good deals on some to buy. They are wholesale to churches, libraries and schools (homeschools count). They have Your Story Hour, Focus on the Family, Jim Hodges, and more.
We do use a lot of mp3 discs I made from downloading for free at Librivox.
I have found many of the Cm recommendations from AO, SCM, and other CM based lists on librivox and myaudioschool.com. You have to specify what book you want since these are books that are at least 80 years old and in the public domain.
I am sure that the others have CM type books too, I just have no personal experience with them. =)