My pen nib from my Wacom tablet was just about worn out, I was chatting with a friend explaining this and where did he buy new nibs from?
He smiled and showed me the secret compartment of nibs, I smiled and said wow I never thought to look there.
I had this Wacom tablet for ages a gift from a customer and friend, I basicaly just been drawing and never really thought about nibs, so before you rush off and buy nibs have a check in the pen pot as your find loads of different nibs, some will be spring loaded, giving a nice soft brush strokes, others are hard and so on.
The metal clip next to the pen or you can find it in the centre of the pot, this is used for removing the nibs by pulling softly outwards, before you do this be sure to unscrew the black area at the end of the pen.
Replacing the nib is best done by hand and not using the metal clip.