Where can I find the source of Nautilus?

If you want use the same sources from the installed version you must get it from the Ubuntu repository.

First, from the softwate center, software sources, you need to enable the "source code" sources.

Then you can use apt-get source, to get the source, and dpkg-buildpackage, to build the package.

Check the following link for details. http://www.debian.org/doc/manuals/apt-howto/ch-sourcehandling.en.html

apt-get source nautilus

Instructions for hacking on the Nautilus code are available here. Instructions on how to get the code and build it can be found on this page.

It seems you don't need to install it and can test it by running it from the source code directory. This means you can keep your installation while testing your modifications.