Hello Colin.
You can get king pin sets from here:- Stainless Steel King Pin Kit (Royal King Pin Kit) 78-3111-R, 78-3111 – Belcher Engineering (belchers.uk).
These are the stainless steel types and they do come at a cost, however there are some on ebay that are the standard steel types etc. Give Belcher Engineering a call they should be able to help you out as they get parts from the USA, let us know how you get on.
Kind regards, Charles Gough