You can the source from the daily snapshot site http://oss.backendmedia.com/snapshots/. Alternatively you can get them from the subversion repository at http://svn.oss.backendmedia.com. You can easily download and update this subversion repository with one of the many freely available clients listed on http://subversion.tigris.org/project_links.html.

The repository is world readable, but write access is limited to people that have shown they know what they are doing.

You can also directly browse the repository with any brower. Keep in mind that you can get hightlighted source code by appending "s" to any file URL (to be exact any file ending with .php, .ini or .tpl).

For example:
http://svn.oss.backendmedia.com/core.phps