Try Perkins Glass and Mirror. I know that they do small special orders. You might also want to try Hy-lite Mirror and Glass, not sure if they replacements, but they could point you in the right direction.
I used onedayglass.com for a piece of Octagon shaped glass. They do custom shapes for glass and mirrors. It was fairly inexpensive. The cutting tolerance is +/- 1/8 inch. The glass was high quality, but their customer service was meh.