Focal Point offers different thicknesses and can cut to any reasonable size.
I'd recommend 2mm for most carriers and 3mm for ULF contact frames.
Schott glass would be quite expensive. You can get optically coated picture
frame glass fairly reasonably, but it's more fragile and its difficult to cut and edge. I recommend the titanium coated type rather than the original
magnesium fluoride Denglas which is no longer made. Don't confuse this with etched nonglare glass. And it needs to be specially cleaned, just like a lens. Don't grab a bottle of Windex. Someone still owes me a lot of
Schott enlarger glass, but they haven't made good. I suspect they just couldn't get it themselves, despite still advertising it. I don't think Edmund
is a good source for this kind of thing, though their big industrial catalog is
an excellent resource for a variety of other useful things. Ignore the hobby
catalog and related website. The world's biggest optical coating lab used to be just across the Bay, and they could custom do just about anything,
but got bought out and relocated.