Here is a previously owned rubber eye cup for your Canon AE-1 Series camera. This will fit all of the Canon A, FT, and almost any other Canon Camera ever made, even the newest digital cameras. Canon was smart and made all of their camera viewfinders the same size.
This is genuine Canon, not a cheap imitation. What's the difference? Well, the fake ones are harder rubber with little to no elasticity. They will not last, will not stay on the camera, and are uncomfortable.
I really like having one on my camera. My favorite thing is that it makes the viewfinder readout easy to see even on a bright and sunny day. Prevents unwanted light from effecting your judgment of the image you are producing. Also prevents eyeglass lenses from getting scratched. Makes shooting more comfortable.