The clicky part of a clicky pen.
If you're looking for replacements, just pop onto Amazon and get some third-party ones. I bought
these for ?3.49 and haven't had any issues. In fact, they fit more snugly that the official ones, which is a plus since the plastic always ends up wearing down over time anyway.
One option that I didn't see before is
this: 99p with free delivery for five styluses! That's the option I'd recommnd if you're ordering from Amazon UK - no doubt there will be similar listings for other countries.