A plain flat copper washer should do the job in theory.
The Nissan part is two copper washer linked together to form both sides of the joint, and these require one less limb to install. Three instead of four.
In previous threads, others have described washers which include a rubber ring within them. I forget the name for those, but as it's only sealing 1bar of water, it doesn't need to be too posh.
If your coolant is leaking close to the radiator top hose, then it should be relatively easy to fit new washers. If the leak is at the turbo end, it may require turbo removal to get proper access to fix it
A crushed/mishapen banjo fitting on the hose could be a culprit if something has been overtightened in the past.