Well, you'd need to find out the package name for whatever runs the top bar. I presume you are talking about the Unity top bar: getting the source for the whole of Unity would be overwhelming, and I'm not sure if all the elements of Unity are all separate packages or not.
To get the source code of a given package you do
(without sudo). You need to make sure you have enabled the "Source Code" option in your software sources, which you can find in the Software Center (Edit > Software Sources)
apt-get source *package*
This question on AskUbuntu will give you a lot of information to think about: http://askubuntu.com/questions/28372...hrough-apt-get
Looking around in the Synaptic Package Manager may help you work out what you are looking for. If you haven't already installed it, it's in the Software Centre or else
. There is a search bar, you could type in "unity" without quotes and see what package descriptions fit what you are looking for.
sudo apt-get install synaptic
EDIT: this question might also help you: http://askubuntu.com/questions/22189...e-with-apt-get