STAEDTLER replacement nibs for... https://www.amazon.com/dp/B07QZ3XZ4Y?ref=ppx_pop_mob_ap_share
Still on the first replacement I used because my OG nib had worn out. Had my S7+ since launch, replaced the original nib about a year and a half ago.
If I were a user who had my nibs break/wear down frequently, I think I would go with an option where the nibs were easy and inexpensive to purchase and replace. I do not know about replacement nibs for the s-pen. I do know replacement nibs for the Norris are easy to get and cost around 10 bucks for a pack. I love my s-pens AND my Norris pencils. I use both depending on how I feel.
The link I am brought to by Amazon says 14.95 ... so here is the link with the 9.99 price: https://www.amazon.com/gp/product/B07QZ3XZ4Y/ref=crt_ewc_title_dp_1?ie=UTF8&psc=1&smid=ATVPDKIKX0DER