They're available through http://www.redhat.com/software/rhel/register/howto/
however, because you're technically not using RHEL, I'd guess that you'd automatically invalidate any technical support agreements. The cheapest AS subscription is $1500 USD, the cheapest ES is $350 and the basic workstation is $180, though there are cheaper academic licenses if you qualify. At that price you might as well just run RHEL and get full support.
From all the documentation I've seen about updating CentOS, they seem to encourage using YUM and CentOS updates are fully supported by the Duke YUM site. Plus it's free. At least with CentOS2, they even openly acknowledged that up2date is not supported.