Subpoena the telephone records from the fax machines, from all area Kinkos's.
Why? It wouldn't show you who actually sent them. It would only show the receiving fax number and the Kinkos fax number.
If the forgeries were done on the Kinkos computers, it can be traced via their login records. But if someone just walked in off the street and asked to fax something, paid with cash, and left, there would be no way to ID the specific person, other than a mugshot.
OTOH, these idiots were dumb enough to forge something... they probably paid with a credit card or check.