4 inch diameter x 1 inch thick pyrex $20
5.6 inch diameter x 1 inch thick pyrex $25
C8 blank 1/4 thick at edge $50
The C8 blank is a Celestron. It was rejected because of blemishes in the polished surface and is f/2. Regrind this to anything less than about f/5.5 and blemishes will be removed and you'll have a good light bucket.