Hillbilly beat me to it. I read the same thing about the insulaters. Got a dump in your area? Look for old tv tube glass. Nice and thick. Like he said if you have limestone in your area, you should have some chert, of some kind or another. Also go to a glass, or window store, or manufacturer, and get the scrap glass. You should be able to get some thick stuff also.
Good luck.
Stickbender.