Most plotters use the old hp plotter pens which are getting kind of hard to find,..,.In the past I have used plain old bic pens with enough vinyl wrapped around them to make it so that they will fit into the blade holder clamp and tighten enough to keep it secure during the plot process. any old felt tip will work also if the barrel of the pen is small enough to fit into the clamp to hold it in place while in use. I've even took an old plastic barrel off an hp pen and fitted a new plain old bic pen inside it and used that. the only thing you have to do is make sure the pen will touch the paper when in it's pen down position and raise off the paper when it is in an up position. The plotter is stupid and doesn't know what instrument is clamped inside the blade holder. if you want to plot very small tiny detailed letters or graphics similar to blueprints you may need to disable the plotters internal offset so that corners will be squared.
edited to add,..I pry the top off of my old felt tip plotter pens and use inkjet printer or stamp pad refill kits to refill them myself,..it prolongs the life until the tips are completley worn down to a nub but until then you can reuse them ten times their normal lifespan,...just be sure not to over fill them as its gonna make a nasty mess,..being a sign painter I often as not use my plotter for patterns as much or more than I do to cut vinyl,.....
Hope this helps ya,....