These inexpensive test cards are available here:
From eBay: BUY NOW Polarization Test Cards
Get some polarizing filter sheets here:
From Amazon: BUY NOW Polarizing Sheets
Explore many aspects of polarization with this kit:
From Educational Innovations: BUY NOW Polarizing Filter Demo Kit
See this short introduction to linear polarizing filters on Hyperphysics, and many more details on polarization from Wikipedia.
Polarization Test Card: light has features that are invisible to the human eye, such as the orientation of its electric field (denoted as polarization), but can be revealed to us by a polarizing filter, used to block light with specific polarization orientations. Inexpensive test cards like this are often found in shops selling polarized sunglasses. The molecular structure of plastics can rotate the polarization of any light that passes through, and this card has such thin layers added to the top of the photo to create the hidden silhouettes seen here.