This piece is 50x50mm, but it comes in different sizes.
I paid about $4 with shipping.
From Wikipedia:
Magnetic field viewing film is used to show stationary or very slowly changing magnetic fields; it shows location and direction. It is a translucent thin flexible sheet, coated with micro-capsules containing nickel flakes suspended in oil. When magnetic lines of force are parallel to the surface of the carrier sheet, the surfaces of the flakes are reflective, and appear bright. When lines of force are perpendicular to the sheet, the flakes are edge-on, and appear significantly darker. When the film is placed on a magnet's pole, the latter case applies.