Hi Kim.
I am a newbie too, but maybe i can help you.
To have the sources of the packages you need to add in your source.list a line like this:
deb-src http://http.us.debian.org/debian/ testing contrib non-free main
Take a look that we are just adding a "-src" in the original line and the rest is intact.
deb http://http.us.debian.org/debian/ testing contrib non-free main
But if I am not mistaking, to use "apt-get source" the package must be a "debian package". So maybe you will need to find the dependencies manually.
I hope it helps you.
Regards.