https://www.thingiverse.com/thing:21911
This is an exact 3D modelling of a N900's stylus, I have used this in the past to print a loss replacement. You could take the model and edit it to let you gue in a nice Fisher universal space pen refill(d1); that is assuming that the refill would fit the slot, just use the back of the stylus as the stylus if you want a pen too.
(edit)I see Wikiwide already linked it, wow creepy how close his wording was I though he was quoting me!