I ordered them from a Perspex supply company who will cut to particular shapes. Perspexsheet.uk. One of the "bespoke" options has the Morgan window shape. You just specify the dimensions. Cost me just shy of £24 delivered for four sawn pieces. I elected to have them all the same size to make getting the fixed window in easier. I ordered 3mm material but it actually had 2.8mm printed on the protective covering. I also found the cut was 1mm smaller all round than I had specified (but I had added an extra 1mm to the height as I saw their terms and conditions stated a tolerance of +-3mm). I did add a thin strip of Velco loop into the fixed window channels. This has a bit of give in it and stops any rattles with the slightly undersized panels. I did need to do some trimming of the sloped edge to meet the frame accurately. I added a "handle" to the moving panel made from some 8mm rod and two stainless domed 8mm nuts. Allows me to open the window from the outside as I have no exterior door handles.
I elected to use Perspex. The company also supply polycarbonate for a few pennies more
