Official nunchuk. Use code PPWII for 10% off (£10.93 - £1.09 = £9.84). Go via TCB/Quidco/another for cashback to make a further saving.
Hut's stated RRP is £14.69 and the cheapest I can find for the white nunchuk controller accessory. Next cheapest is a black nunchuk for £9.95 at John Lewis (or £9.99 at HMV - cashback).
I don't have a black Wiimote so this suits me best and may do you, too. Next cheapest white is £11.73 at Amazon.co.uk.
Hope this helps someone!
(Adding keywords: nunchuck [to help it be found for those who spell it that way!])