You may try finding parts from damaged (or not needed anymore) jewelry, or visit other similar artisans that have spare of such small pieces which you could use it. Another idea is start making simple work on (brass) screw heads, just cutting and some grinding and polishing. Finally, roll stoppers can be made from the same material as the pen, wood, acrylic, you should have some leftovers. Just some fine and careful cuttings and finishing, then gluing on the cap and you're done. The stoppers' shape is limited only by your imagination. You'll find more satisfaction by doing yourself, instead of finding them ready to use.