Amazon.com also has new ones, but only the black appears to be in stock (
https://www.amazon.com/gp/aw/d/B01637XE4W/ ). Currently the Oasis 2 covers have the same shortage - only the charcoal is available on amazon.com nothing on amazon.co.uk. Until amazon.co.uk has any battery covers back in stock, you might have to get an external usb power pack.
That amazon.com still sells them brand new indicates that they are at least not discontinued.