If it is convex round, can try a local clock repair shop. or you could try Norkro Clock company on the web. I am not sure of their web address.
If it is convex square, you could try the above, but they might not have it.
If you need holes in the glass, a bit of valve grinding compound from an auto parts store, and a brass rod of the approximate size of the finished hole from a hobby shop.
:I have restored a Bendix Clock Radio Model 753 from about 1953. The clock face and dial face (glass) has benn broken. I am looking for the glass only, if someone might know where I could find it.