Where can you find the Ubuntu original source code? Umm…. Linux isn’t quite like that. Linux is a kernel written by and maintained by Linus Travalds. On top of that is what we call GNU (a collection of thousands of Linux-compatible programs), which work with the kernel to make the software we commonly call “Linux/Gnu” such as in distributions known as Ubuntu, Linux Mint, Arch, etc.
So when you talk about the Ubuntu 14.04 original source code, you’re talking about a collection of thousands of different programs written by thousands of people interfacing in one package or distribution called Ubuntu. The good people at Ubuntu weave it into an interface that is relatively easy to install and use. But there is no source code that makes up what we call Ubuntu. It is thousands of programs included with the Ubuntu distribution and packaged into a downloadable ISO file you can burn and install.
From there you can do just about anything you want by adding or subtracting thousands of additional free programs and giving your system the look and feel you like.
You can contribute to any of the many aspects of the Ubuntu project. Just go to their website at Ubuntu.com and volunteer. This answer was typed on an Ubuntu system. I’ve been using Linux since the year 2000.