Hi phill. Not sure where you are, but here in the states I was able to contact Microsoft Support and complain about how quickly the pen nib was wearing down and they didn't hesitate to send me 3 nib replacements for free. They do wear quickly, but happy they sent the replacements. I imagine at some point they will start selling them, but for now, contact MS support.
As a follow up to my post above, my replacement pen arrived today from the Netherlands, took 3 days not counting the weekend. It's great but I would have been happy with replacement nibs - strange that they sent me a £44.99 replacement instead lol