Nikon Corporation supplied photographers a wide range of eyepieces to mount/screw on the viewfinder of the camera, enabling to focus without wearing spectacles. The eyepieces made for the various rangefinder cameras are extremely rare, thus difficult to find and pricey. For nearly all Nikon SLR cameras and some compact cameras Nikon provided eyepieces with a diopter value from -2.0 up to +3.0. All standard eyepieces on Nikon cameras have a diopter value of minus one (-1), so don't look for an eyepiece with that value. Nikon engineers considered minus one as the optimum starting point for the majority of SLR users with normal sight. This implies that if you are replacing the standard eyepiece (0.1) by a neutral correction eyepiece the diopter value will be zero!
Most modern digital and film SLR cameras have a built-in diopter correction feature. See the manual of your camera (most correction buttons are on or near the viewfinder).
For a clear overview of the eyepieces made for Nikon SLR cameras see this Nikon eyepiece application reference.