You can make that yourself. An ordinary piece of 3/4" plywood coated with marine grade epoxy resin will be waterproof and is what many people make plywood aquariums out of. I recently acquired a 300 gallon tank with a broken bottom pane that I am in the process of replacing with plywood.
As far as the risk of the bottom of a tank cracking, as soon as you add a substrate it will protect the bottom and cracking will not happen. Even dropping a large rock on it will not matter. The only reason the bottom pane was broken on the tank I got was because it was dropped during relocation, and I chose plywood simply because a glass replacement was over $700.00.