You can obtain the source code of the Tape Database easily via Subversion (SVN). If you have a UNIX-like OS, SVN is possibly already installed. In this case execute the following command in the directory where you want the sources to go to:
svn co https://zephyrsoft.svn.sourceforge.net/svnroot/zephyrsoft/trunk/tdb
If you are running a different OS, refer to your SVN client's manual and use this login data (list of SVN client programs):