..or you need a converter foor a pen that uses non-standard cartridges. Make one up! I did this for a vintage Aurora DuoCart. I cut a silicone sac to size and used shellac to secure it to a cut down cartridge. Other adhesives can be used (do not use cyanocrylate adhesives like Superglue).
Simple solution, it takes five minutes and it works great. More complicated versions can be made, but this one works, is easy to make and it is cheap!