Create a Fork, where to get the source code?

[DarkPenquin]

Lets say I use a remastering tool such as Reconstructor for Ubuntu, Revisor for Fedora or InstaLinux to create a bare bones distro I can then add to/customize the distro. I use Synaptic or what ever built in software downloader is bundled with it to add to it and customize it. - Where do I get the source code for each and every app so I can make it available under GPL?

Whats the easiest way to do this? Is there an app the scans your system and tells you a list of apps you need source code for?

This would be a good idea if there is not such an app. It would also be nice if it searched the repos and downloaded all the source code you need so you can put it on a site for distribution.

But otherwise.. how is it generally done? I don't want to modify any code itself, just assemble and customize a distro using existing tools, so I won't mess with code at all, but I still need to be able to make the code i use in the end product available for download.