This seems to have popped back up again on a different URL (sneaky!).
I'll pop my previous bumph in...I've been looking around for another genuine Xbox battery for the boys controller. I purchased one myself when Amazon had them for £11.99 and now need another. The price at Amazon has gone back up to £16.14.
Student Computers have them for £9.99 delivered but they're not in their original packaging. They are advertised as '[i]x-display and un used cancelled orders
' and in 'an As New condition' with a 12 month guarantee.
They also have the genuine Xbox One Micro-USB Controller Cable
if want to add to your order as a backup for £2.49.