For a genuine Samsung replacement part for the Galaxy Book4 S Pen, you can try contacting Samsung directly through their customer service or visiting a Samsung Experience Store or Studio. However, I believe you cannot just buy the tips and will have to get a whole new pen.
Thanks but when I know it's only the tip that needs replacing why would I want to buy a new pen? Surely when most companies are trying to be environmentally friendly it makes sense that you can buy a replacement tip. Even apple sell replacement tips. The pen is only 6 months old.