*If they do have the returns then it would have to come from the IRS directly and they should scout and scalp that person.*
By my reckoning there are 4 possible sources, the IRS, Trump's hired accounting firm, The Office of the Assistant Legal Adviser for Ethics and Financial Disclosure or Trump/family.
I doubt the IRS leaked anything.
I sorta doubt the accounting firm.
The Office of the Assistant Legal Adviser for Ethics and Financial Disclosure would be the more likely source. I'm sure there are plenty of never Trump bureaucrats there.
Trump/family maybe, especially since there was no there there in the NYT's initial story, but that could change assuming the NYT's actually has something.