Want to Buy Type C / Type M focussing screen for Nikon F4

I am interested in buying a Type C (or alternately a Type M) focussing screen for my recently acquired F4 camera to use for photomicrography, where the light is usually too low to use an ordinary screen. (My light microscope dates from 1938 so the lighting relies on the use of a mirror.)

I live in England.

What do you use for a light source to shine into the mirror, I have an older freestanding light source that came with a 60 watt bulb, now it has a 100 watt halogen bulb in it. No need for a special screen in the camera. ND filters are almost needed for the eyepiece on my vintage 1920 B&L.