Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

You could make it yourself, and if not contact a local glazier, and get him to cut one for you in the correct thickness.

Made a couple myself, back when i collected Wilesco. They were made from unidentified scrap glas, and has held up fine.
However, later wisdom forces me to recommend a confirmed borosilicate glasstype, as it has a very low thermal expansion.