It used to be posted on his home-page (and I think the wikipedia article used to link to that?) But I couldn't find it on there any more. It's available on most abandonware sites though (use the Mac version with a mac emulator, the MS-DOS version has some important bugs in it).

[edit] The source code for it is available still on his homepage, but You'll need paschal and assemblers for MacOS to build it I imagine.

http://www.erasmatazz.com/library/source-code/index.html