Haven't heard or tried any of the aftermarket ones, but you could roll the dice. Here's the original, not cheap.

http://www.amazon.com/Samsung-Stylus-Galaxy-Note-Packaging/dp/B014R9DMZ4/ref=sr_1_3?ie=UTF8&qid=1447972764&sr=8-3&keywords=note+5+s+pen

EDIT: These claim to be OEM.

http://www.ebay.com/itm/Original-Samsung-Galaxy-Note-5-Stylus-S-PEN-for-AT-T-Verizon-Sprint-T-Mobile-/181878132232?var=&hash=item2a58c80e08:m:m9XFNHHM_S5tutOxtA_SxKA