> I'm writing to the ATM list instead of astromart because what I'm looking
> for is closer to this group's interests. Does anyone have a spare 12.5 inch
> flat or a lead to where I might get one?
If you can't find anyone who'd be willing to part with such a blank, and if you don't want to pay the full price to a commercial source, you might want to check out firms that specialize in security glass. Here in Finland many amateurs have purchased window glass and borosilicate blanks from such a source. I have no idea what the cost might be where you live, but it might be worthwhile to find out. The catch is that the glass is of course undocumented, and not intended for optical use.
Jaakko Oksa