I got lucky and picked up a second hand HP-7475A for $50 about five years ago. It's the GPIB version, not native RS-232, so I also had to find a way to connect it to a modern PC. Ended up going with a GPIB<->USB adapter from AliExpress (about 10x cheaper than one from Prologix) to plug it into an RPi, which I can use to send HPGL files to get plotted.

Also got a friend to hack away a chunk of the plastic casing with a circular saw so I could use pen adapters for modern Sharpie and Stabilo markers, which are much easier and cheaper to acquire than old format plotter pens.