Another option is to pick up a few things from a local hardware store and make a custom size one yourself.
If you have a Lowes or Home Depot nearby you can pick up a window screen kit that has the side rails, corner pieces that fits into each rail. Kits may or may not include the strip of rubber that goes into the rails to hold the screen but can be found there as well. Then you would just want some screen to fit in that is metal not fibrous (aluminum works well). This is what I did on a 75g aquarium that I use for my dubia colony.