Pilot no longer seems to offer the original model, which I insist on, but I was able to acquire large quantities from several Amazon stores.
Any B5 notebook with squares, blank pages or dots will do (Leuchtturm ones are nice), as long as the pages are numbered. I have laptops and paper notebooks everywhere, so I do not use one linearly until it is filled, but hop between them. Each entry is dated, though, so this does not cause a problem.