All you need to get the source of the app you want is make sure that you have a deb-src matching the deb line in your sources.list.
Code:
## Unstable Sources
deb http://ftp2.de.debian.org/debian/ sid main contrib non-free
deb-src http://ftp2.de.debian.org/debian/ sid main contrib non-free
Then apt-get update and and usually you should make a directory that you change into when getting the source with
apt-get source glade this downloads into the directory you created if going to build the package then you will want to do
apt-get build-dep glade. I never used glade so have no clue what you need to for that part.