£35.99 at Wiggle, both top cap options available Wiggle | Cane Creek 40 Series ZeroStack 1 1/8 Inch Headset | Headsets

Your fork will determine if you need tapered or not, quick look at CRC indicates there aren't any non tapered 650b forks, so you will need to look at the tapered headset options( i.e. not the ZS44).

Tapered forks aren't the future, they are the now, if buying new today, can't think of a single reason not to get tapered.

Knowing which frame you have would be useful, saying an On-One is too vague, too many different models & specs.