NASA cleaned out their optical vault and had a cache of vintage Pyrex and other blanks in various substrates.  These are all in good shape, although some have minor scratches or scuffs from handling or testing.  No cracks or chips, are are in good shape.  Shipping costs between $20-$40 on average depending on which size/weight.  I have a good method for packing these for shipping and have sold over 30 with no damages reported on any through shipper.

Ask any questions about individual substrates.  There is Pyrex, Fused Silica, Ohara E6, And Shott BK7 available.  Most of the pyrex are slightly pre figured by Corning and have a 2mm raised lip on the edge, the others are all flat.

There is a $10 handling fee for shipping materials.