You can get the source from several places. Each package has its own page on the Debian web site (at packages.debian.org), where the source can be downloaded via a link at the bottom (a few lines under the Download button).
The other way, if you have a Debian system installed, or at least use apt-get, is to use the command "apt-get source <package>". (i.e. apt-get source dpkg) >Where I find the source of de Debian GNU/LINUX