Howto Installing FreeBSD 9 source
1. Download the tar file that matches the version you installed.
Unpack the archive to /usr/src:
# tar -C / -xvzf src.txz
Be aware that this will only get you the code for FreeBSD-9.0-RELEASE, not anything that has been added since then.
SVN (Subversion) is a newer source control system. It is not part of the base system, so install devel/subversion. Then check out the source:
# svn checkout svn://svn.freebsd.org/base/releng/9.0/ /usr/src
Other versions will have different URLs, like svn://svn.freebsd.org/base/stable/9/ for 9-STABLE. svn includes a larger history and can take up to twice the amount of disk space as a CVS checkout.
See the Handbook SVN mirror list for alternate servers which may be faster for you.