Hi All - I am clearing out my parents loft (eBay here I come :) ) My dad and his dad were almost fanatical photographers and almost 100 years of family history are tied up in old negatives and slides. Here is the problem - many of the negatives are 120, 126, quarter plate and half plate. I can't find any transmissive scanner quoting anything larger than 35mm. We lost dad over a year ago, I would love to transfer the pictures I have found on to DVD so my mum can have them.
You could try photographing the slides with a digital camera with a good macro facility. I have used this technique successfully using a Nikon 5400 which can focus down to about 2cm. The inage can be reversed (negative to positive) automatically so you can retake the photo if you are not satisfied with the result.
The light box can be a lamp with a diffuser OR a laptop computer screen showing a blank sheet of paper in a word processor program to form a white background. If you use this latter method, move the slide/negative slightly away from the screen and use a wide aperture on the camera. This is to force the screen to be out of focus to hide any imperfections/marks/pixels. You can then concentrate of the sharp focus of the picture.
I have the above scanner which scans 5x4 inch negatives It has it's on light in the lid. But I don't know if they are still made, I've had it for over 3 years. A light box will do if you can spread the light evenly over the negative.
The final solution was to use the lightbox - I tried using the laptop screen but suffered from moire patterns. The ideal light source turned out to be a mini fluorescent under a new white polyurethane kitchen cutting board as a diffuser. Am now saving up for a proper scanner £2000+ OUCH!!