Greetings Ash,
In the past I have found Fowlers of Bristol to be excellent but I think that you may find such genuine items out of stock at most dealers
The next obvious is to try ebay
After that try, a local to you, radiator repair specialist who should be able to instal a core into the original headers otherwise it might be a case of trying to modify some from another model of Suzuki or marque.
Sorry I can n't point you to a known source, maybe somebody more knowledgeable will be along soon. Good luck and please let us know how you get on.
TTFN
Hugh.