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RickM
The only thing I've not been able to find there is borosilicate glass. I'm having an absolute nightmare trying to get a 250mm round piece. The only place that has it is in the US and charges $40 + $30 shipping with no returns (so if it's broken, I'm screwed).
It doesn't need to be borosilicate. Try a piece of round mirror glass like this: [www.amazon.com] (that's 10 inches diameter, which is 254mm). You should be able to source one in your country pretty easily. Where are you located?
Many people (including me) have used mirror glass like that with much success. The thermal stresses aren't great enough to really risk breaking the glass, and if you do manage to chip or crack it, it's cheap to replace.