Redhat relases the source RPMs for RHEL to their public FTP site. Luckily for you
some nice people have gone and made 'rebuild' distributions where they've re-compiled these source RPMs to make a (free) distro that's almost exactly the same as the real RHEL distro (the main difference is they have to strip out all the Redhat logos from the installer packages and stuff like that to avoid violating Redhat's trademarks). The one that seems to be the most popular ATM is centos (www.centos.org
). Centos 3.6 is the equivalent of RHEL3 update 6 so that's probably what you want since its the latest update.