This is done by editing a file (/etc/apt/sources.list) and adding the repositories of packages you want.
I don't use Ubuntu, so I haven't test this, but this page proposes these edits to sources.list
for your version of Ubuntu.
There's also the option of using Synaptic, it must be somewhere on your system, if I remember well it lists all the available packages and I think you can search for packages using keywords.
If it's not, install it from a terminal (apt-get install synaptic).