In the National Archives, the Bill of Rights is in the center of Archival Secure Wing East.
On your way up you can find the The Bill of Rights on one of the floors in the fenced off area with a safe (next to a Lying, Congressional Style). You can also find the Magna Carta in a similar room northeast from where you found The Bill of Rights. Both can be sold to Abraham Washington; 100 caps for the Bill of Rights (125 with a haggle), and 75 caps (100 if you haggle) for the Magna Carta.
After this item is looted, it may not ever be dropped - the only option to remove it is sell to Washington. It is flagged as a quest item. Oddly the Magna Carta is not flagged as a quest item, and can be dropped.