Incoming links: Reality (2023 film)
"Sources and methods" seems to be an official CIA/NSA terminology to talk about the main categories things that whistleblowers were not supposed to whistleblow.
It came to Ciro Santilli's attention in the Reality film about Reality Winner, and are words she apparently used extensively and which are quoted e.g. at www.theguardian.com/us-news/2022/jul/25/reality-winner-leaked-file-on-russia-election-hacking-because-public-was-being-lied-to
The term is more broadly used however outside of the NSA, e.g. it is also present on this website which talks about the CIA 2010 covert communication websites fiasco.