Try Elmer's Aquarium. They don't ship tanks but they may ship the cover. Item number 529654
Go to their tanks and kits page and look at your tank type, the cover is usually listed in the kit's bill of materials on it's own line.
Also if your glass just cracked or broke, a local glass shop should be able to make you a new piece of glass for it the same size and shape, and even temper it and sand the edges for you.