The ROM images on /usr/share/qemu-kvm/rhel6-*.rom come from the
Red Hat Enterprise Linux 6.4 package gpxe-roms-qemu-0.9.7-6.9.el6.noarch.rpm.
 
The source code for those images can be downloaded from:
http://ftp.redhat.com/pub/redhat/linux/enterprise/6Server/en/os/SRPMS/gpxe-0.9.7-6.9.el6.src.rpm
 
For more information on how to obtain source code for Red Hat Enterprise Linux
software, you can refer to Chapter 1. Obtaining Red Hat Enterprise Linux of the
Red Hat Enterprise Linux Installation Guide.