My 1947 4 X 5 Super D Graflex does not have a fresnel lens since this wasn't available at this point. I would like to know if I can place of fresnel lens on top of the existing ground glass, which would be very easy to do. More importantly, I want to make sure that there wouldn't be any focus shift. On my Linhof, the fresnel is behind the ground glass, while reversed on my Graflex products. If I wanted to get even more deluxe, could I simply add a later bright screen on top? This would be an easy solution w/o any adjustments.

Where is the cheapest source of fresnel and/or bright screens?

I don't know a good source since Edmunds was split up.
My Super-D does have an original fresnel which is very heplful so I think its worth some effort to find one.
It may be that one of the current makers of screens, like Bill Maxwell, may be able to supply one. Do a Google search to find him.