from fileconnect then these do require a S/N. If you haven't got this you may need to speak to your reseller or Symantec Customer Care: [[EDIT: as stated by revaroo - something came up half way through writing this post!!]]
A serial number is required to initiate the software download through FileConnect. This serial number is specific to the FileConnect site and is not the product license key. The serial number will either come with an upgrade or purchase letter.
If this information is not available, a serial number it can be procured from:
Symantec Licensing Portal:
https://licensing.symantec.com/acctmgmt/index.jsp
or request a FileConnect serial number through Symantec Customer Care:
http://www.symantec.com/enterprise/support/assistance_care.jsp
from: http://www.symantec.com/business/support/index?page=content&id=TECH63533
There were, however, some full client binaries available for certain clients e.g. x64 2003/8 installations:
http://www.symantec.com/business/support/index?page=content&id=TECH71200
unsure if there were others (e.g. 6.5.2A)?
***Another EDIT***
I'm sure you're already aware that 6.x is soon to be EOSL, but just thought I'd mention it!
https://www-secure.symantec.com/connect/forums/be-aware-netbackup-6x-end-support-life-fast-approaching
