Do you mean the aquarium cover or the actual frame supporting the glass? I know you can find a number of replacement aquarium covers on Amazon (I've bought a couple) but I assume you would need two of them (do you have support frames down the middle of the tank wide enough to attach two covers to it? Many have lights, some do not. A few have hinged tops but those probably won't work unless you can get one big cover for the whole tank.
If it's the actual frame for the glass - I have no idea. If you know who the manufacturer is you might contact them - I have no idea how it could be done if there were fish and water in the tank without the tank collapsing but you know your tank better than anybody.