I have done, just cutting a disk from an older pair of glasses. There's one stuffed into the eyepiece of my Hasselblad and it's very useful if I'm prepared to keep my glasses off the whole time I'm shooting.
You can buy them in different sizes if you know the diopter you need, but old glasses, a Dremel and a diamond wheel is not too difficult as the lenses are usually plastic. Just try to use the same eye lens you would normally focus with and cut from the centre area.
