How exactly did he get the memo into the hands of the professor? He EMAILED it – therefore, subpoena his email accounts (all of them) and the professor’s email accounts.
My guess is you will find three important things (1) what leaks took place historically going back in time which may be years of leaks; and (2) who exactly the professor is leaking toward (NYTIMES by journalist name for example), and (3) then subpoena the journalists emails which may take years of court battles which would then yield the other leakers to those particular NYTIMES folks.
This is what I think happened:
By the end of their first meeting, Trump knew that "something" was wrong with Comey.
He put a Security Detail on him. It led to the Professor and then to CNN.
And then Trump fired him...
Two days later, it was confirmed. Comey was one of the leakers.
The next day, the 12th, Trump tweeted the "you better hope there are no tapes".
Right now, I'd say Trump has BOTH sets of tapes.
We shall see.