Most pool stores stock it. Pool Repair Epoxy or something similar. Other stores like H-D MAY have it, but they should epoxy putty which is pretty similar. You cut off equal amounts and knead it together until it's an even color and then apply it. I've used epoxy putty to fix things (successfully) like a car's heater core. But pool epoxy putty is designed to be APPLIED under water, as opposed to being applied dry and then immersed after it's dry.

Most car parts stores stock epoxy putty. It is NOT Bondo.