Where can I find the source code of Ubuntu?
How do I obtain the source code for Ubuntu 12.04 LTS i386 32-bit?
There are multiple ways to get the source code for Ubuntu.
You can browse to the online archive archive.ubuntu.com
You can download it through apt
If you want the kernel, I'd start at http://www.kernel.org/
The problem with wanting the 'source for Ubuntu' is that the distribution is basically made up of a bunch of different packages. I'd probably narrow down to what packages you want to look at and go from there.