I will soon have all the parts available for some kits of the wireless KTRL gamepad.
If you want to build one of these, you will need to get a 3.7V 500mAh Li-Po battery (20x40mm) for it from another source, like this one:
https://www.ebay.com/itm/232948355016 or
https://www.ebay.com/itm/332830330735.