Here's the really important point: The key to this news story is a single piece of information among literally thousands of numbers and individual data points. Whoever leaked this information knew enough about taxes to know what to leak and no more.
If I was going to make a guess: someone really close to Trump who was intimately familiar with Trump's finances and who also didn't have a professional code of confidentiality.
If You Only News initially suspected it was Trump's daughter, Tiffany. Then they got a little more plausible with Hair Furor's ex, Marla Maples. Josh Marshall didn't offer any names but provided background, quasi-legal and personal, and No More Mister expanded on that.
It really doesn't matter who leaked them; Trump is over. Finished. Kaput. Stop being afraid he's going to win. He isn't. He can't. He has eliminated every single pathway to an Electoral College victory, almost all of them by his own hand.
The Russians are not going to hack the election. (They are simply unable to do so. This is Democratic propaganda.) Women and Latin@s and squishy Democrats who only vote every four years will turn out in droves to vote against him. Every voter living in a non-battleground state -- there are at least forty of them, and their number will increase as this election turns into a landslide for Clinton -- is hereby emancipated from the scare tactics of every Jackass from Obama on down to vote their principles in order to make sure Madam President gets the message: use the next four years to achieve real progress on the issues you say we can't, or else get primaried from the left and be a one-termer.
Clinton won't have an opponent so weak, so awful, and so self-destructive next time.