The DarwinBuild sources are available via anonymous CVS access using CVS' pserver protocol.
For (t)csh:
# setenv CVSROOT :pserver:anonymous@anoncvs.opendarwin.org:/Volumes/src/cvs/od
# cvs login
Logging in to :pserver:anonymous@anoncvs.opendarwin.org:2401/Volumes/src/cvs/od
CVS password: [return]
# cvs co darwinbuild
For bash, zsh:
$ export CVSROOT=:pserver:anonymous@anoncvs.opendarwin.org:/Volumes/src/cvs/od
$ cvs login
Logging in to :pserver:anonymous@anoncvs.opendarwin.org:2401/Volumes/src/cvs/od
CVS password: [return]
$ cvs co darwinbuild