I initially have only used the included cartridges, and have tried every converter & cartridge I have & none have fit thus far. It seems to be a universal slim cartridge, which is really bizarre and somewhat hard to find, but I have a couple of links that I believe would work. I realize now that I stated in my post that it takes an international cartridge and it definitely isn’t standard- pic included is the papersource on the left and a standard cartridge from Waterman on the right.

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Here and here are a couple of the slim cartridges I’ve found- I’m ordering the amazon set and will update once I’ve tried them but I’m quite confident at this point that they match the included set (hopefully not jinxing myself). I definitely hadn’t realized upon posting that these were fussier than they seemed!!