@GeorgeR
Have a search for 'quadcopter battery' on Amazon (or your marketplace of choice) and you should find one of the right size and correct connector. I've bought a couple of batteries and charged them with the 'Timer Power Unit' for various projects.
WARNING!! BEWARE! I've found about 50% of the batteries I've bought had the wires the opposite way round to that required by the M5Stack controllers. They will instantly kill your controller! Triple check the polarity before ever plugging them into your controller.