You can download Drupal 6 from two sources:
Drupal.org: This is where you can get the latest release of the open-source Drupal 6.
Acquia.com: Acquia is a company co-founded by Dries Buytart (the founder of Drupal) and Jay Batson. Acquia has produced a corporate version of Drupal, creatively called Acquia Drupal. Acquia Drupal has some of the better modules pre-installed and provides some great extra services like uptime monitoring, version updates, and advanced support to your installation and modules. Downloading and installing it is free but the services do cost a bit extra—starting at a few hundred dollars per year. That's well worth the extras you get.
Note
Are you running a corporate site and you're not quite up to speed on Drupal? Consider signing up for Acquia's support services. Acquia Network subscriptions provide commercial-grade support and network services for all Drupal 6.x web sites (not just Acquia Drupal) to help you implement Drupal with confidence. Visit http://acquia.com/ for more information.