Keep an eye on Ebay, they show up from time to time. I bought a couple in 5x7 format thinking they would work for sheet film. Well they do to a point, but the film flops around inside the holder as the film is a lot thinner than the glass. Still they are well made and look very nice!
I doubt if you can modify a standard film holder as the the glass would be too thick.
Come to think of it Kodak still makes glass plates to special order. I wonder if Kodak Technical/Customer support can point you in the right direction?