See #11.
In any event, I can think of two people who are known to be involved in this affair who would fit the description: Bill Burkett and James Moore.Separately, Rather has also described his source as "unimpeachable". In my opinion, this description is unlikely to be affixed to somebody as unstable as Burkett -- even by one as partisan as Gunga Dan.
Ergo, James C. Moore, move to the front of the line...