Kaweco International Fountain Pen Cartridge
Kaweco's international size ink cartridges work in a variety of fountain pens, including all the Kaweco fountain pens listed on this site.
To use: Simply unscrew the back of the fountain pen, pull out the empty ink cartridge, attach a new ink cartridge and you can continue writing.
One package contains 6 standard cartridges with rich, consistent Kaweco ink. (I mean, they've been doing the whole pen thing since 1883, so I think they nailed it.)