I did find a copy lurking online… sadly not an ISO.
http://archive.kernel.org/redhat-archive/redhat/linux/5.0/en/os/i386/ -- you will find boot.img and supp.img floppy images in the images/ sub-directory.
The catch is, the installer does not support downloading over anything other than (possibly active) FTP or NFS… and that mirror does not support anything but HTTP. `wget` refuses to download due to `robots.txt` telling it to go bugger off.
QEMU's 'pcnet' network interface is recognised by Red Hat 5.0 though, as is the IDE interface, so if you can clear the FTP hurdle, that should get the environment up.
