apt-get install does not download the source codes of packages. There is a specific
apt-get option to do that which is
source causes apt-get to fetch source packages. APT will examine the available packages to decide which source package
to fetch. It will then find and download into the current directory the newest available version of that source package
while respecting the default release, set with the option APT::Default-Release, the -t option or per package with the
pkg/release syntax, if possible.
Source packages are tracked separately from binary packages via deb-src lines in the sources.list(5) file. This means
that you will need to add such a line for each repository you want to get sources from; otherwise you will probably get
either the wrong (too old/too new) source versions or none at all.
So the syntax to download the source package of something to the current directory would be (no need to be
apt-get source <package_name>
Also note that as the
man pages says you need to have the
deb-src repositories enabled for all the regular
deb repositories you want to keep track of source packages. The simplest way to do that would be to just add
-src to the original repo's initial
deb http://archive.ubuntu.com/ubuntu/ trusty main restricted
deb-src http://archive.ubuntu.com/ubuntu/ trusty main restricted
On a different note, you might have checked the
/usr/src directory which is the designated place for all kernel sources.