Came across a Rolleiflex Automat 6x6 Model RF111A (S/N: 600409; late 1937) in very good condition. Everything works, shutter speeds sound good, but the ground glass was broken.
A piece of the original remained, and measured 0.05" thick.
I'd try Rick Olsen. A quick search will find his screens. I have also heard than some have made their own from old Mamiya RB/RZ screens. I expect you'll have to adjust focus a little. I have the same year Automat and love it. Good luck.
A few years ago I bought a Chinese split focus screen off Ebay intending to use it in my Yashicamat 124. Instead I fitted it in my Microcorkd (British Rlleicord clone) it's very bright significantly better than the original Microcord screen or the screen in my Automat. It as about $30, they are still available at roughly the same price.