NPC make a Polaroid Back for the FM/FE series of cameras. Cost is about 650 USD from B&H for a new back or around 450 USD used.
They are useful if you are working in a studio environment and require an check of lighting levels and angles for example. They are also used for some field applications at a professional level such as architectural work with perspective control lenses.
Polaroid backs aren't cheap to run though, often costing upwards of 2.50 per frame.