5.9 Where is the source code?
Source code is included for everything in the Debian system. Moreover, the
license terms of most programs in the system require that source code
be distributed along with the programs, or that an offer to provide the source
code accompany the programs.
The source code is distributed in the pool directory (see What's in the pool directory?, Section 5.10)
together with all the architecture-specific binary directories. To retrieve
the source code without having to be familiar with the structure of the FTP
archive, try a command like apt-get source mypackagename.
Some packages are only distributed as source code due to the restrictions in
their licenses. Notably, one such package is pine, see Where is pine?, Section 4.10 for more
Source code may or may not be available for packages in the "contrib"
and "non-free" directories, which are not formally part of the Debian