In the museum, it is at the end of a hallway that leaves the main gallery and passes the statue room and the security room.
To enter it, take the underground tour then apply for the job at the Museum. Once you pass the painting test, take the Forgery Training and you will get a key to the Supply Room.
Get the paper
In the supply room
the supply room is in the mussem on the first floor to your far left
Don't ask me, o_o.
you get them when you get a job there. Talk to the assistant curator
finish the tests and he will give u a key.
Once you pass the test by the assistant curator, and finish the Forgery training, he will give you a key to the Supply Room.
Apply for the job with the assistant curator and when you pass the Forgery Detection class, he will give you a key to the Supply Room.
you can with the key!
The same way you got in to get the picture piece : use your key.
You get it when you pass the Forgery Detection training with the assistant curator.
Once you pass the test by the assistant curator, and complete the Forgery Training, you are hired and receive a key to the Supply Room.