I went on a deep google search for NGK spark plug caps the other day and thought I'd share my findings.
I ordered some NGK 8384 SB05EP Spark Plug Cap from ngk.com. I prefer the ones that clamp directly to the electrode without the nut. 16 still in stock
The P version means "with special waterproof cover". Don't know if that will cause any fitment issues, but we'll see.
I also ordered NGK 8080 SB05F Spark Plug Cap from ngk.com. Just in case the P version mentioned above has a fitment issue. 165 still in stock.[You must be registered and logged in to see this link.]
(chart to help decipher the NGK codes)[You must be registered and logged in to see this link.]
(complete chart to help decipher NGK codes)
Searching by stock number helps sometimes with some retailers (found on complete chart above)[You must be registered and logged in to see this link.]
The boat websites seem to have older bike plug caps.
I have no relation with or knowledge of these retailers. Deal at your own risk.
The caps seem to be out there, but dwindling due to discontinued.
Good luck! Leave some for me!!!