A second possibility is to download the source code from upstream directly instead of downloading it formfrom Debian repositories. This has the advantage that if you not only want to read the source code but might like to change something you can directly commit and submit it to upstream (assuming it is not a Debian patch).
You can usually find out the upstream source code repository URL in the file /usr/share/doc/$package_or_program_name/copyright.
$ head /usr/share/doc/git/copyright
Format: http://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
Upstream-Contact: [email protected]
Source: https://www.kernel.org/pub/software/scm/git/
Files: *
Copyright: © 2005-2014, Linus Torvalds and others.
License: GPL-2
Files: xdiff/*
Copyright: © 2003-2009, Davide Libenzi, Johannes E. Schindelin
This file is also referenced from packages.debian.org (search for "Copyright File").
