Drupal - Where can I download old releases of Drupal?

You can get Drupal 6.20 from the Drupal 6.20 released page.

You can also view a list of all current and previous releases on the Releases for Drupal core page, complete with download links. It goes all the way back to Drupal 4.2.0, which I'm now off to install to see what happens!

I know you've already got it answered... but Git wasn't mentioned.

git clone http://git.drupal.org/project/drupal.git target_directory
cd target_directory
git checkout -b local 6.20

Good option if you just want to cut to the chase and "git" it. Best paired with "Drush", to download, update, backup etc from the command line.