A few months ago I made an important discovery: international long and short standard ink refill cartridges fit the Daiso brand fountain pen.
This is great to know because Daiso stores are pretty inconsistent as far as what they stock regularly. What’s there one week might be gone the next, never to be seen again. My local Daiso store has had their fountain pen in stock pretty consistently for the last year or so, but it’s always sort of a surprise. I always expect it to be gone.