Wish you had told me that 25 pounds and 10 years ago…
Carter Page and/or Manafort and/or Pap
Stefan Halpner is another possibility, even though he wasn't a campaign member. He met with multiple members including Flynn, Pap, and Page.
Brennan >>Hakluyt/MI6 >>> Halpner
That ensares CIA, MI6, and the FBI and would explain their reluctance to release it.