Submit FOIA yourself.
I think I found the place to make the request. Since the Hatch Act is handled via OSC (Office of Special Counsel) and McGahn was Special Counsel, then that is where the letter will exist.
https://osc.gov/pages/foia-resources.aspx
I think Q was giving us a hint by linking us to the pamphlet
https://osc.gov/resources/ha%20pamphlet%20sept%202014.pdf
Text template for email:
Send to: FOIARequest@osc.gov
Subject: FOIA Request
This is a request under the Freedom of Information Act.
I request that a copy of the following documents [or documents containing the following information] be provided to me: A letter sent by senior members of the Democratic party outlining potential 'Hatch Act' violations regarding an anonymous online user named "Q".
The letter was sent to Don McGahn.
The letter was sent early October this year (2018). Since Mr. McHahn formerly departed from his position as White House Special Counsel on October 17, 2018, I believe the estimated date range of this letter to be between October 1, 2018 to October 17, 2018.
In order to help to determine my status to assess fees, you should know that I am a individual civilian with no affiliation to any organzation and I am making this request for personal use and not for commercial use.
I am willing to pay fees for this request up to a maximum of $100. If you estimate that the fees will exceed this limit, please inform me first.
Thank you for your consideration of this request.
Sincerely,