Backup Exec 15 requires a valid license key. The ISO itself is the installation media. You cannot legally download or use it without an active entitlement or a purchased license from the past.
Essential Guide: Symantec Backup Exec 15 Download and Best Practices Symantec Backup Exec 15 Download Iso
The download link was a direct FTP. No HTTPS. Just ftp://backup-files.old.symantec.biz . Backup Exec 15 requires a valid license key