I need to make some pre preg sheets from a sheet of glass but type of glass should i use ?

I have used standard window glass. Even used old double glazed units (but remember to drill a few holes in the material holding the two pieces together so the expanding air can escape!

I have also used mirrors, as these are very flat.

I have had no issues up to 140 deg C using a 3 deg C per minute ramp cycle.