Knight Optical Ltd: http://www.knightoptical.com
"Proper" polaroid sheet - HN22 - is no longer manufactured, but this company have a supply.
It's quite a thick and fragile material so you'd be best to get them to cut it for you. A lacquered finish is available for scratch protection.
With crossed-polaroids of HN22 the background will be completely black to the eye so this is the polaroid to use for photography.