I am looking for a product that I used in college to produce B&W transparent prints in a darkroom. It used the traditional dev, stop, fix process and the final image could be backlit. I have done the webresearch but with the billions of ink jet and digital options I’m guessing it’s a needle in a haystack situation. Any help?
Chances are the material you used is no longer made. However you could just use film, assuming you are working from negs with an enlarger in the normal way. In particular if you can find an orthochromatic sheet film this would be suitable. Ilford make one which is available in sheets up to 8x10 generally and 10x12 by special order. Because it's orthochromatic it can be handled under (dim) red safelighting so it's not a complete pain to use. It will probably be very fast compared to paper, but it is designed for copying, so it's reasonably slow (they quote ISO 80: I'm not sure what paper speeds are, but I suspect significantly lower than that).
It's going to be an expensive experiment.