I second Tracy's input..... some other "handles" you'll hear it called is "Fireplace Glass" or "Pyroceram"....... I have a local glass outfit that calls it "Fireplace glass"...... I get mine there simple because it's usually not offered in the sizes I need/use, and if I buy it there, they cut it to size for free. Even with that, it's usually between $24-$32 per piece, but unless I do something dumb, like drop the platen on the shop floor and break it, the "glass" lasts a LONG (and in years) time for me.
However, if you have platens that are "standard" sizes, it's best to purchase from folks like Tracy/USA Knifemaker supply.....so you know you're getting the correct stuff.