For Cooper, any of the names in this article, IRAQ: A Question Of Trust - The CIA's Tenet takes the fall for a flawed claim...... is a good starting point.
For Miller, I have some guesses. She may have been the first reporter to tell Libby about the Plame connection. I believe Libby said he learned of it from reporters. If she did that information would help prove his claim.
She also could have told other reporters. Third, she might have been working on a piece about Plame. In that piece Plame and Wilson might have been sources, they wanting to extol Plame like Wilson in the press. Her sources might have had it in for Wilson. Tenet and his people would be primary suspects. It gets lost in the fog of lefty blather about Bush "lying" about WMDs, but Bush didn't "lie", he was supplied the info from the CIA. Tenet was chagrined about Wilson's article.
One has to first assume that the multiple sources are all genuine live human beings, and not a fictitious device or composite the reporters used to cover their own behinds.
Going with the assumption that the sources are real, we need to understand the anonymous-source speak reporters use to hint at why they consider the source credible. There is a subtle descending hierarchy.
The best anonymous source is one that can be characterized fairly specifically, as in a White House source, or a State Department source, or a police department source -- that sort of thing -- versus the more broad administration, or government, or congressional source.
In addition, an "official" (direct) is better than a "source close to" (indirect, 2nd hand or maybe an eavesdropper), which is better than a "source familiar with" (maybe 3rd hand or snuck a peak at some documents). A "senior official" is better than just "an official," who is better than just "a source in; close to; familiar with..." And so on.
So, assuming Novak followed this inside baseball source-speak, I take his "senior administration official" who is "no partisan gunslinger" to mean something like a career upper-level manager working somewhere in the executive branch, but not in the White House. I take his "senior administration official" with no other characterization to mean someone who may or may not be an appointee versus a careerist, and again working somewhere in the executive branch but not the White House. (Most of the time, "senior" means someone at assistant deputy level or above. If you see a characterization similiar to "high administration official," that usually means at the level of a Cabinet secretary, or #2 in a department, or chief of staff, etc.)
The next hints Novak gives us are in this sentence: "He said Wilson had been sent by the CIA's counterproliferation section at the suggestion of one of its employees, his wife." That's pretty arcane stuff. Not even a Cabinet secretary would know that unless a report happened to cross his desk or it was mentioned in a national security briefing -- something like that. Depending on how tight an operation the CIA Director ran, he might not even know about it.
At least one of the people to whom Novak spoke was a male who either had access to fairly arcane national security and CIA personnel information, or to documents that discussed the matter. The man is not likely to have been privy to the information otherwise, unless he worked for and had close contact with a true senior official such as a cabinet secretary and/or the department's #2 person -- and not unless the secretary/#2 received a report about the matter.
Even then, it couldn't be just any cabinet secretary, but one who either had direct knowledge of CIA personnel matters and/or received national security reports/briefings that included the information. So, depending on the specific subject matter, the list would most likely include the cabinet-level offices of Defense, State, Justice, Treasury, FBI, CIA. Within Defense, it also could include the Joint Chiefs and military intelligence.