Allegro.pas's source code is maintained in a SVN repository. You can get clients for most platforms at https://subversion.apache.org/, and there is good documentation.
To get the current WIP version, use the next command to:
svn co svn://svn.code.sf.net/p/allegro-pas/code/TRUNK allegro-pas
Note that only developers can commit new content to the SVN.
To get older versions, look at the RELEASES subdirectory:
svn co svn://svn.code.sf.net/p/allegro-pas/code/RELEASES/<version-num> allegro-pasWhere <version-num> is the version you're looking for (eg: 4.4.2, 4.4.3, etc). Note that not all old versions are available. You can find those at the downloads page.
You can get, also, the unstable version 5 using the next command:
svn co svn://svn.code.sf.net/p/allegro-pas/code/BRANCHES/5.0 allegro-pas.5
You can browse the source code online at http://sourceforge.net/p/allegro-pas/code/