One of my daily duties is to run a cash register at my work, this involves having people sign receipts when they pay with credit card. I swear, people steal pens on purpose. Those pens just disappear. Granted, we employees do walk off with them also, but that's a different matter entirely...
PensRUs.com has a smart idea, as do other competing companies:promotional pens.

Pensrus.com can provide you will all the pens you need. This site is an excellent supplier of pens and has a whole litany of writing instruments to choose from. If you need a pen they can supply you with whatever you need. 

They are super cheap and they have one of the biggest selections that I have seen. They seem high quality and I might buy my dad a few so that he can see the quality. I hope he likes them!