Red Hat Enterprise Linux 7.6
download a 4GB dvd image from redhat: rhel-server-7.6-x86_64-dvd.iso
move it to a linux system, e.g /root/rhel-server-7.6-x86_64-dvd.iso
Centos 8
Download a 7GB dvd image from http://isoredirect.centos.org/centos/8/isos/x86_64/CentOS-8.1.1911-x86_64-dvd1.iso
create a bootable usb flash drive:
insert usb flash drive into a linux box
using dmesg, note the mount point,e.g sdf
findmnt /dev/sdf
Copy image to usb: dd if=/root/CentOS-8.1.1911-x86_64-dvd1.iso of=/dev/sdf bs=512k
after the prompt comes back use it for installation