The first step will be to correctly identify the " square cutout " as to what type of receptacle it is . Hopefully it is a NEMA standard receptacle .
As long as it is not a proprietary receptacle , I have no doubt you can find the power cord at »www.digikey.com/
Have you tried getting the cord from the manufacturer of the charger ?
If nothing else try to get a good closeup picture of the receptacle with a good ruler held up to it .