I picked up a packet of something similar from Super Cheap awhile ago. They had them in varying sizes.
I say similar because they were smaller and used on my 3rd brake light enclosure on the HJ Galant. Of course the holes weren't damaged that they went into. Did the job and looked great except slightly different colour but no-one has noticed.
You should be able to get something similar from Super Cheap as Wayne suggested. Otherwise most of mine is held on with cable ties in the holes I can use cable ties, and I've got just enough to use in the holes I can't fit cable ties into.
SA.. when you're back in Japan.. might buying 5 dozen of it?! then sell it here?! cause the ones that's sold in supercheap or autobarn is usually smaller in diameter or they've ran out..