You can get older releases (back to Moodle 1.3) from the following URLs:
http://download.moodle.org/stable13/
http://download.moodle.org/stable14/
http://download.moodle.org/stable15/
http://download.moodle.org/stable16/
http://download.moodle.org/stable17/
http://download.moodle.org/stable18/
http://download.moodle.org/stable19/
http://download.moodle.org/stable20/
Anything older than that could be fetched from CVS.
Please note that anything older than Moodle 1.9 is unsupported. As far as I know, 1.8.14 was the last release in the 1.8.x series, and no further fixes -not even security fixes- are expected any more.
Saludos.
Iñaki.