Possibly the best professional true glass (not acrylic) hand-made screens are available at this link (and you will not find a nicer person to deal with than Steve Hopf):
Hopf Ground Glass
But making one yourself is also not at all difficult. Click on this link to see an article written by a fellow APUG member who has posted do-it-yourself instructions:
Making a Ground Glass Focusing Screen by Ian Grant
The word is that Ian's method produces excellent screens.
It's also worth mentioning another fellow APUG member 'laser' (Bob Shanebrook) who has posted classified ads in the past (most recently on 7/24/12) offering original equipment replacement fresnel screens for Graflex cameras that already have a working ground glass. I can vouch for these personally as I purchased a beauty from Bob for my own 4x5 Pacemaker Crown Graphic.