Yeah, fortunately pens are relatively easy to find on Amazon, eBay, and a few online stores (just Google 'HP Plotter Pens'). My Roland and HP plotters use the same type of pens, btw.

All USB-Serial cables and adapters I’ve used so far have worked (including Arduinos!), but you might need a DB-9 to DB-25 null modem to physically connect your adapter to the plotter. Biggest thing to watch out for is to make sure you get an RS-232/Serial version of the plotter and not an HPIB/GPIB version. The easiest way to check is to look for a DB-25 connector on the back of the plotter instead of a Centronics-style connector (looks a bit beefier).

EDIT: Btw, some plotters (like the HP 7550A) will have both types of connectors on the back. In that case you’re good!

The only reason I didn’t buy that HP pen plotter is because I’d want the freedom to give it any kind of pen or marker, not just ones made specifically for it. I imagine it’s not possible?

You can make your own adapters depending on the pen you want to put in it. I've made some out of dried out HP pens that I cut the ends off and jammed a Sharpie into. I've seen nicer 3d printed adapters on eBay and Thingiverse as well. When using a heavier pen like a Sharpie, you might want to set the plotter's speed a little lower to avoid knocking the pen loose when it accelerates.