Malcolm, Unforunatly There are no sources mentioned in the particular chapter on the North Berwick witches.
I have looked it up in my copy of the Dictionary of Witchcraft by David Pickering and he wrote that it was taken down in a statement from Agnes Sampson [one of the witches] during her interrogation.
I wonder if there is a mention in any of the state papers for Scotland as King James VI was present and questioned Agnes personaly. James also went on to write his Discourse on Demonology later.