HP Plotters are a lot of fun, but the pens for these plotters are all expired and an absolute nightmare to refill, though it is possible.

Walmart as it happens, stocks a very cheap assortment of fiber pens with the right colors for the task. Some very quick modeling later, we have some replacement shells which are designed to fit those pen nibs and a portion of the ink carrier. The nib, and rear the of the pen is removed with a pair of scissors (the best way I found to get them out without damaging anything), and the ink felt is tapped out of the shell.

The nib is pushed into the new shell, the ink felt into the rear, which is then trimmed and cut flush, after testing I put some clear tape over the rear of the pens to try to prevent them drying out. They fit into the original carousel exactly the same as the originals, only these ones have ink in them and not dry sticky powder. The dirty caps on the pens did contaminate them, but the rubber seems to be fairly soft and flimsy so I don't know how they would stand up to being cleaned. The results aren't perfect when the demo attempts to shade in large blocks of color, but otherwise I couldn't be happier.

The total time to print 8 new shells was less than 2 hours, it takes only a couple of minutes to disassemble a walmart pen and get the pieces into the carrier, and the total cost for the entire construction was $6.66. I would like to make a version which has a screw on top, and probably printed out of resin to try to avoid the pens drying out as fast (these FDM prints are liquid permeable to some extent so they will not last as long as you might think), but for now I'm very happy with these, the print file used follow, they need support around the rims and to be 100% infill.