AFAIU debian-installer currently contains only documentation, where is the actual code for the Alternate CD installer?
The alternate installer is created by Debian (upstream from Ubuntu).
Thus to look for the source, you'll need to pull this from the Debian repositories.
According to debian:
Debian are transferring to Git - but the alternate installer is not completely migrated yet.
Two steps are required - checkout using
sudo apt-get install subversion, git, mr svn co svn://svn.debian.org/svn/d-i/trunk debian-installer
Then pull the git repositories:
cd debian-installer/scripts scripts/git-setup mr -p checkout
Go for a coffee or two - each individual component (package) is checked-out - it will take quite some considerable amount of time over a slow link.
All the components are installed under the sub-folder