You probably already have spare mesh caps. Spares ship with the product. They are easy to overlook.
Okay, a little more detail now, and with some more keywords to help people who are searching for this info ...
The Apple In-Ear Headphones with Remote and Mic product ships with two extra “mesh caps” included. They are in a small, transparent plastic capsule embedded in the cardboard folder containing the documentation. They are easy to overlook, and probably some customers accidentally throw them out! I know I almost did.
I lost one of my mesh caps and it took a fair bit of searching and scratching my head wondering how I was going to replace it before I noticed a reference to the spare mesh caps in an Apple KnowledgeBase document (
http://support.apple.com/kb/TS2452).