Now that www.sheetfilm.be has disappeared, are there any current recommended suppliers for ground glass? - I'm thinking about a 6x9 roll film back for my Walker Titan SF 4x5 so potentially looking for a 4x5 glass with 6x9cm markings.
My first thought would be that there are many APUGers who make their own ground glasses. And it is not difficult to do by the sounds of things. I have not yet tried it myself but if you were game a short search would pop up lots of recent results. There have been lots of discussion on here of late ont he subject.
I am unfamiliar with professional resources for ground glasses.
There are some, they use ground glass in optics labs for diffusion. You might just google around, I did a couple years ago when I broke the glass on my 5x7 Arca, luckily Bob Watkins had a new Arca glass on eBay and my problem was solved. Arca parts can be very scarce...EC
The ground glass on my De Vere monorail needed replacing when I bought it back in 1976, I just put grid lines etc on it using two Chinagraph crayons, I sharpened them to a very fine point first. Still looks as good 34 years later. The advantage here was I could use blue for the grid and red for the Format frames.
I've done the same with the screen for my Quarter plate camera for a 6x7 back. There's an easy way to get the lines in the right place once you have the back
Your best option otherwise is ask Mike Walker himself, yes known for his excellent service.
I just made one from a Diycamera kit. Works fine took about 15minutes, cost was minimal. I was going to buy the glass locally & mail order the grinding compund & realize d it would be less expensive to go with the kit. Between driving around for the glass( a mile?) & searching for the grit & time....
I got a 5x7 groundglass from Steve Hopf. I would not hesitate to buy another from him. He is very responsive, ships quickly, and provides great value. I don't know if he will put 6x9 markings on one for you, but you can contact him here: Dead Link Removed.
I also have used Steve Hopf and thought it was perfectly satisfactory. He ships USPS priority, and even internationally, a 5x7 should fit in a flat-rate envelope and ship for only $13.