USB-C to USB-A cable approved by Benson Leung (check him out if you haven't already). He's a Google engineer, and spends his time testing 3rd party USB-C charging cables to see if they are safe. Worryingly, quite a few aren't, and could fry whatever you plug them into. This cable is on his list as being approved. Granted it's not the fastest at data transfer, but if it's just charging you want it for, this is great (supports up to 3A). Currently £12.99 on Amazon.
Belkin USB-IF Certified 2.0 USB-A to USB-C 6ft cable £5.99 instore @ Maplin (+ £2.99 del if order under £10)