See Is This How Avenatti Found Out? - Talking Points Memo.
Also this ...
2. Where did Avenatti get this information?We don't know for sure. But it seems to have come from original bank records, which are usually only disclosed in court cases. Avenatti's document listed the exact dates and amounts of the money transfers--the sort of information that banks and individuals usually guard closely. One theory is that Avenatti obtained the information as part of discovery in a California lawsuit in which he, as Daniels's lawyer, is trying to get a judge to free his client from the nondisclosure agreement that she signed as part of her hush-money settlement with Trump, arranged by Cohen in 2016. Another is that the information was leaked by someone who has access to banking records filed with the Treasury Department.
Seven Questions Answered About Those Payments to Michael Cohen - John Cassidy - New Yorker
In response to your post, it just reinforces my original suspicion that the source is Mueller. I doubt any private institution would take the risk of being uncovered and risking liability. Robert Mueller has built his life on proving himself to be the Witch-king of Angmar. His investigation leaks like that boat that stranded everyone on Gilligan's Island.
Thanks for the pithy research! :^)