a 6x12 piece of woodstove glass is $30 at onedayglass.com (just the first google hit). That is the good 1200 degree ceramic stuff.
I am not sure if a fireplace door is that good of glass, but they can be had for $5 or less at garage sales and habitat re-stores (goodwill/salvation army for building materials).
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Good advice R Scott. Point of information: fireplace glass doors are almost 100% tempered glass. I've never heard of ceramic being used in a glass door set. Tempered glass will abruptly and dangerously shatter when overheated. Customers have told me it sounds like a rifle shot, with glass pieces landing 15' - 20' away. Not a good situation if the kiddies or pets are in the way. I'm not sure what toaster oven glass is...it may even be like the Corning ware glass, which is not an approved material for what you are describing.