On debian (derived) systems, the bash completion scripts are bundled in the package bash-completion:

qwurx [shmem] ~ > dpkg -l bash\*
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/T
+rig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                   Version          Architecture     Descripti
+on
+++-======================-================-================-=========
+========================================
ii  bash                   4.2-5ubuntu3     amd64            GNU Bourn
+e Again SHell
ii  bash-completion        1:2.0-1ubuntu3   all              programma
+ble completion for the bash shell
un  bash-doc               <none>                            (no descr
+iption available)
[download]

Don't know how that is handled on systems with the RPM package manager (RedHat, Fedora, SuSE, CentOS etc.).

You could grab the debian package, make a temporary directory and unpack the package file there with

ar x ~/Downloads/bash-completion_2.0-1_all.deb
[download]

You will find a data.tar.gz which contains the files, unpacking that (tar xfz data.tar.gz) you will find the directories etc/ and usr/. Reolcate their content to the system root. The sourcing code for the completion files is in /etc/bash.bashrc as follows:

# enable bash completion in interactive shells
if [ -f /etc/bash_completion ] && ! shopt -oq posix; then
    . /etc/bash_completion
fi
[download]

I guess that on non-debian systems the setup is similar.