The problem is likely that there aren't spares available. For some reason companies are notorious for locking up whiteboard pens and only giving new ones when you've signed some obscure form in triplicate.
I feel irrationally responsible. I used to steal them out of the stationary cupboard in high school during the 1980s and give them away to anyone who wanted some, and was surprised at how popular they were with other students. At the time I thought that either everyone else liked art class as much as I did, or that perhaps some of them were using them for graffiti, but some years later it dawned on me that they provided a relatively safe way to abuse solvents :-.
