I think we are confusing the simple split pin for the front bearing with the ferociously expensive cotter pin on the rear hub lower swivel pin...
A wheel bearing kit should come with a new split pin - diameter 2mm, length 5.5cm - available at good accessory shops. If in trouble I can order Nissan genuine items - not £25...
BTW, I have limited stock of the expensive rear cotters at £6.99 + VAT - came by a small batch at a keen price. When these are gone, they will be £20+ again!