The IRS has all the records that were filed.
Actually, no, not after a certain number of years. They’re pretty aggressive about reducing paper files to database entries and shredding the originals.
Also, the grounds on which Mueller, as a federal law enforcement officer, could subpoena Trump’s returns on an ex parte basis under 26 IRC sec. 6103(i) are fairly narrow and require that there be a specifically designated federal criminal statute under which he is proceeding, a showing of reasonable cause to believe that a specific criminal act had been committed, reasonable cause to believe that the return may be relevant to the commission of that act, and that the information sought cannot be reasonably obtained, under the circumstances, from another source.
I’m not sure if all of these elements could be satisfied, although it’s entirely possible that Mueller has information the rest of us don’t that ties Trump to a specific federal crime with respect to which his return information would be relevant.