Legit plotter pens for my HP DraftMaster are either old and dried out, or new and expensive. Print these adapters and cut down the ink cartridges of normal pens and pencils to fit.

Bonus: you can use interesting inks like white and metallic. Uniball Gel Impact are quite nice, and Sharpie has some nice metallics as well.

If you find other writing implements that would benefit from an adapter, send me the info and I'll make one up.