Some government agency ...
Question: Some government agency provides proof of Santa Claus' existence in Miracle on 34th Street, but I can't for the life of me remember which one it is.
Answer: The Department of Homeland Security… just kidding. It's the U. S. Post Office that recognizes Kris Kringle (Edmund Gwenn) as "the one and only true Santa Claus" in the Christmas classic Miracle on 34th Street (1947).