The note can be found in Ella Ames' bunker. It is on the bottom bunk in the bedroom on the left side of the eastern hallway.
Charles Owen Walker, I better not see one more I.O.U. from you around this bunker. You may be a grown ass man, but I will tan your hide if I have to.
- Mrs. Ames
(also known as your elder who you damn well better respect)
The game editor name for this item suggests this note was written by a person named "Stephanie", whose full name might have been Stephanie Ames (Ella Ames' mother).