It's not clear to me what source you are asking for. Python is installed by default on Ubuntu and the 2.7 version is installed on Ubuntu 12.04 (and I believe earlier versions). Is it python 2.7 that you installed with synaptic? Ubuntu uses CPython, so named because it is written in C. Therefore its source is not put on your computer unless you ask for it. medigeek has posted a helpful answer that will tell you how to download it.
Ubuntu uses python for many of its packages, for example ubiquity (the Ubuntu installer), Jockey (Additional Drivers), and Startup Disk Creator. Since packages written in python are distributed in source form and compiled on installation, you can examine these packages on your system. For example, jockey has source code at
/usr/share/pyshared has python source for lot's of packages (almost 300 on my system).
The source for python 2.7 itself can be found at http://hg.python.org/cpython. Other versions of python have had their source imported onto Launchpad. You can see them here. Click on one you want to see and you can then click "Browse the Code".
Good luck learning Python.