It is the last part of the island, when you have discovered the stolen painting and get kidnapped by the Black Widow. Get past her guards and use the key card to begin the final battle.
it is located in the black widows lair
first you get kidnapped. it is easy from there
it is in the statue room
Get the paper
Use the key card to open the treasure room and battle the Black Widow. (see related questions)
Find out for yourself
In the supply room
In the Statue Room.
In counterfeit. There is a lady with black hair, a ponytail and should be in the museum. I have a ponytail on my character but I got the ponytail in a common room.
In the upper left of the museum.
in the lighted room on the second floor
the supply room is in the mussem on the first floor to your far left