This is not an unusual problem. Most (and perhaps all) fountain pen kits use the 'standard short international' cartridge, or the equivalent 'standard international' converter. The problem is that the cartridges sold at ordinary stationery stores are usually proprietary cartridges that fit the main-stream pens that they sell - Mont Blanc, Parker, Cross, etc. And the proprietary cartridges won't fit in any pens other than those for which they were specifically made.
I'm a big-time fountain pen user, and my solution is to use converters (aka 'pumps') that can be refilled from bottled ink. That way, I don't have to shop for cartridges, and I have a much larger selection of ink. And ink in a bottle is less expensive than ink in low-volume throwaway plastic cartridges.
But cartridges do have the advantage that they travel much better than bottled ink, and they can be a lot less messy to use.
Most of the pen kit suppliers sell both no-name international standard cartridges and international standard converters. And you can find all kinds of suppliers on the internet. Diamine and Private Reserve are both reputable ink (only) manufacturers who package their product in international-standard cartridges that are sometimes found in pen shops.
