The glass shop is the place to go. They will suggest the best glass to use for your purposes. Just know that if something scratches the glass, it cannot be buffed out...usually. But for a glass top, go to a window/glass store that only does glass.
As to shopping for a circular table with a glass top, search the internet and figure out what size you want and get an idea of prices. Then go check out furniture stores and thrift shops.