I need the NBU agent for Linux.
for which version of NetBackup and for which flavour of Linux and version?
You must be having NetBackup license certicate of your organization's there is serial number for NetBackup
Navigate to https://fileconnect.symantec.com and put it there and download the package you want.
If you don't have cerificate, sync up with your symantec sales rep, or symantec partner to arrange it.