The main reason we do not recommend running old unsupported releases is that they will not automatically get security fixes.
For bash releases 10.04 LTS, 12.04 LTS, and 14.04 LTS only will get a security update und upgrading.
Adding a random ppa to an old release may introduce even more severe security issues in case you do not know the maintainers of this ppa. In addition, many ppa will often also not offer packages for old unsupported releases.
However you can always downlaod and install any package from it's Debian package, which is available for download from official ressources. Only in case debendencies can not be resolved from a newer release to an old release we can not do so. For bash it should be safe to download and install from a newer downloaded Debian package, as dependencies are the same in new, and older releases (but possibly not in very old releases).
From the link below select your release on top of the page (preselected for trusty), then select your architecture (amd64 or i386) at the bottom of the page for downloading a Debian package of bash from security.ubuntu.com.