Some of you may remember this thread -

I know for a lot of people the bane and slayer arrows as ingredients are extremely annoying because there simply is no way to intentionally go out and get these ingredients like you can with say bruised spore pods, LDS, or demon blood.

I think the answer to this lies in the new monster manual release.

My proposed solution is to add an NPC who is a purveyor of a wide assortment of monster lore and tracking/hunting equipment (like slayer/bane arrows). However, this NPC won't deal with just anyone, he is looking for individuals who have shown an interest and ability to track down a wide assortment of monsters.

In short, he is looking for someone who has unlocked a certain amount of the deeds in the MM. Now the details on if this should be account or character deeds, how many deeds, and of course if he will just sell the arrows or have a barter interface that requires you to bring him (intentionally farmable) ingredients in trade to make the arrows, are all entirely debatable.

The main point is that the Monster Manual and its focus on a wide assortment of monsters makes it the ideal way to introduce this sort of thing. Not to mention the fact that it makes a fantastic amount of sense as you will have a tremendous number of rangers hunting down the various critters for access to the assorted bane arrows to keep in their quivers.