Here's an additional trick. When you find a cartridge, the diameter of which fits the pen, but it's too short - don't give up. I simply slide an appropriate length of rubber eraser from a mechanical pencil up into the end of the barrel, and use that as a spacer to keep pressure on the cartridge so it remains tightly in place. Many vintage Sheaffer erasers were about an inch in length, and approximately the required diameter. They just need trimmed to an appropriate length.
In addition, I've found that some Esties can use 2 currently available "universal" cartridges, if one is pushed on to the section nipple, and the other is reversed and similarly used as a spacer, up in the barrel. Using this method allows you to keep writing even after the first cartridge runs dry.
- - - Good luck, Jim