This one may be just a reminder for myself when I forget but it may help someone else as well.
If you download the CENTOS 6 Linux netinstall image it will prompt you for a URL to the install files.
Here are the links.
for 32 bit: http://mirror.centos.org/centos/6/os/i386/
for 64 bit: http://mirror.centos.org/centos/6/os/x86_64/
Hope it helps.