These are absolutely ESSENTIAL for a poison build; do not leave home without them.

Can be purchased after giving the Shrine Handmaiden the Hollow's Ashes which can be found in dark firelink where Yoel/Yuria would normally be. Posting this since neither Yuria, Yoel, or this page mention the Hollow's Ashes option and for people who are already pvp capped this may be the only way to unlock poison throwing knives after ascending up another NG+.

47 physical base damage with a D scaling in STR and a B scaling in DEX, +25 base poison with a D (almost C) scaling in LCK.

This page needs to be updated. You can get poison throwing knives without having to become hollow if you go to the Champion Gundyr bonfire, go to the dark firelink shrine, and go to where Yuria of Londor is normally at. You get her ashes, turn them into the shrine handmaiden, and boom. You can now buy poison throwing knives for 120 souls a piece.

You can get them from the handmaiden as well for the hollows ashes.