If you want to use ink cartridges in your pen it will either require an international standard type or a manufacturer's specific model. Below are lists of each type to help you out for our most popular brands.
International Standard Cartridge brands - these can be swapped with each other
Pelikan (available in long or short, check your pen's length)
Kaweco - *Most Kaweco models will only take short length
Waterman (available in long or short, check your pen's length)
Brand Specific - they can ONLY be fitted into their own brand of pen