Petco, which sells the largest number of 40b in the country, can order glass tops. They are listed in their online site.
Big box glass is not the safest material to use for glass tops. It is sold, with disclaimer stickers on the stock shelves, for above ground floor window repair. In my opinion <--- MY Opinion, it's too thin to span the width of the tank without added support of some sort. Any glass shop or hardware store that fixes ground floor windows can sell you thicker, safer, glass - cut to fit.