I know the rubber eyecup is similar to several Nikon eyecups, like the F65 and D90, but I'm not sure if they use the same style diopter. If you can find a place with some of the square diopters for Nikon, and see if they match up, then you might find an alternative. Other than that you would need an eyeglasses place to make a corrector for you.
The Nikon rubber eyecup is called the DK21 and fits numerous F bodies and early D bodies. There is a square diopter that matches. Check which bodies fit the DK21, then see if they have diopters available. Some camera stores do not stock these items, or only list them for specific bodies.