The Computer History Museum in Mountain View, California, is not just a museum of hardware, but also of software. The Museum has made public such gems as the source code for MacPaint, Photoshop, and APL, and now code from the Apple II. As their site reports: 'With thanks to Paul Laughton, in collaboration with Dr. Bruce Damer, founder and curator of the Digibarn Computer Museum, and with the permission of Apple Inc., we are pleased to make available the 1978 source code of Apple II DOS for non-commercial use.

http://www.computerhistory.org/atchm/apple-ii-dos-source-code/