I mentioned this last year on the black linux mailing list, but you can
get the NeXT-relevent Plan 9 sources from here:

http://www.prim.demon.co.uk/plan9.tar.gz

Downloading the full distribution appears a bit of a pain as you have to
actually install it first from a boot floppy, and it looks as though you
need Plan 9 to unpack it.

Dave