I have 2 spherical mirrors from some computer chip inspection equipment. They are full thickness pyrex in machined aluminum cells.
I took a quick look an a foucault tester, and they appear nicely spherical. They appear to be about f/2.
The coatings appear very good, with a few sleeks and some fingerprints, but very usable.
One has an edge chip, not to the surface, and appears to have a dime-sized stress fracture just below the surface, but neither defect has an impact optically. The second unit is perfect.
$40 for the perfect one, $35 for the chipped, including shipping.