The test is usually done by placing the sample between two polarizers with a light source on the far side. The polarizer nearest the light is called the polarizer because it polarizes the light source. The polarizer after the sample is called the analyzer because it measures how the sample changed the polarization. This arrangement is called a polariscope. If you search for polariscope you will find several you can buy. Microscopes will often have a built in polarizer and analyzer which can be popped into the field of view. I would guess this may be the most common case for your muscle fiber studies as they probably require magnification. More precise instruments designed to measure the axis of polarization of a monochromatic light source are called polarimeters. Often the analysis can be made more complete by including a retarder. Instruments which measure the full polarization ellipse by automatically rotating the analyzer or polarizer or varying a retardation are called ellipsometers. Among these instruments you will find a wide variety of linear or circular polarizers and retarders. At some level it doesn’t really matter what kind of polarizer is used so long as it polarizes the light.

However the simplest and most common polariscopes in the visible usually use a polymer film where the polymer has been oriented in one direction to create a linear polarizer. In good polariscopes the polymer film is on a nice flat glass substrate. However, two pieces of polaroid film (polarizing plastic sheet, not camera film!) work well.

The basic idea is that if you cross two polarizers no light will get through. However, stress in the sample produces birefringence (retardation) by the stress-optic effect. The amount of light that gets through the second polarizer is related to the amount of birefringence in a nonlinear way, but, to some degree anyway, generally more is more. In turn, the amount of birefringence is proportional to the amount of stress. So, placing a transparent object between two crossed pieces of polaroid (polarizers, not camera film) on top of a light source gives a high contrast map of the stress. If the light source is white, you get color bands. If it is monochromatic you get intensity bands. In quantitative measurements the intensity of the bands can be related to the birefringence and the birefringence can be related to the stress.