If you are in London, have a look at the Arc Euro Trade website for details of their products - stepper motors and drivers. Mine are the 220 Ncm ones - fairly large to drive a metal turning lathe system. you could probably try the small ones, or the next up.
Bear in mind that gearing comes into focus here. If you are willing to sacrifice a little bit of speed, and gear down your motor then you can get away with smaller drives and smaller motors. Mine are geared down 60,000 to one, and could still manage 40 ins per minute. This sounds a lot, but the motors require 200 pulses to turn 1 rev, the drives multiply this by 10 micropulses (=2000), I have 3 to 1 gearing to drive my leadscrew, which turns 10 time to move the table 1 inch (i.e. 60,000 pulses). This means that my computer has to put out 60 pulses to move the table by 1 thousanth of an inch - accuracy is what I need, but I also get the power from the motors - more than enough to snap of 6 mm drills without even trying.
I would try the small motor, with the small driver, and maybe some sort of toothed belt driving the pen holder. Bearing in mind if you use a tooth belt system you can alter the various drive wheels very easily until you reach the ideal set up.