I've got a feeling they generally stopped doing them in the 80's or early 90's, plastic lenses are much easier to manufacturer, are lighter, cheaper and easier to stock.I havn't had any for a long time.
IIRC plastic lenses are basically injection moulded to a preset value for the prescription (so the factory might have thousands of each value as a lens blank), then cut to fit the chosen frame and given a final polish, glass lenses had have a lot more processing between the initial blank and final fitting and were far more likely to fail at various points in the process.
I think there is also an element of they can affect the correct value by using different plastics so you can get a higher change in a thinner (thus lighter) lens.