Does anyone know of a good source for 400 x 400 mm borosilicate glass? Right now, it looks like I’ll have to get something custom from Alibaba, wondering if there are some better suppliers.
McMaster Carr is where I got mine
I checked McMaster, but unfortunately they only have up to 12"
Oops, I didn’t convert the mm in my head very well 
http://www.cgindustrialglass.com/ request a quote. Minimum order is $50, thus, 1 sheet would be $50 minimum, price breaks if you buy more.