Jason,
First you need to determine where the leak is located. From my experience, the leak is probably located at one of the connectors where a line enters or exits the tank, and if so, it can be repaired by replacing the connector at a plastic fabrication/welding shop. However, if the tank itself is damaged, it can also be repaired or rebuilt by a plastic fabricator. Since the tank was spun molded, and there is probably no chance of finding an exact match because the mold no longer exist, you can use an off the shelf tank, but you will probably end up with a smaller tank to get something to fit in the space.
Gerald