You won't find anywhere (legit) to download NetBackup 6.5. That version went end of life about 5 years ago (October 2012) so is no longer available. Most of the support docs for this version have already been archived.
Your only chance would be to see if your local Veritas office has an old copy lying around. Any particular reason you need a version that old, and not the current supported versions of 7.7, 8.0 and 8.1?
Hope this helps,
I would look at upgrading ASAP if you are able.
As far as I can see, you have two options (I've just come up with another that may be possible):
If you really need NBU 6.5 then you can open a support case and request the assigned engineer to upload the 6.5 agent for you.
Support can only provide you end of support software but they wont be able to assist you on the issues that arise with the end of support products.
Hope this helps.