When I installed Ubuntu 12.04, I was dismayed to see that the "Software Sources" was gone.  I've already had a beef with Ubuntu since Unity came around (what is already back in the day in the technological world) so that is why I hopped on the 12.04 bandwagon in the first place.  Daily reminders from the system telling me my "Super OS" 11.10 needed to GTFO. So here we are wondering where our "Software Sources" went in 12.04.  Problem solved rather quickly.  I'm not sure if running Gnome 3.x is helping the situation or not, perhaps cos the "alacarte" package was already installed. So go to your terminal and install "alacarte" if it isn't already there.  When it done installing that lovely package open it up however you fancy on doing such things, and you will see a window popup that looks reminscent of the old Windows start-menu or the Ubuntu menu when it was on the top of the screen. Top menu you see is Applications and underneath it (unless you've been fucking with it already) should start something like: Accessorties, Debian (italic), Education (italic), etc.. Go down to the **arrow** next to the System Tools icon (cogwheel) and expand it.  Click **Preferences** and you will see an entry for **Software Sources** and if you check it, it will allow you do as you once had in your prior Ubuntu glory.