Sleep darts?

Sorry if this is the wrong section for this, new to the forums and I had a quick question.

I am just starting a campain with a friend of mine as the DM. I have a thief character who prefers to avoid combat if possible. I had the thought that she could have some sleep darts so if she comes across a couple guards she could knock them out from afar with some sleep darts instead of killing them.

I looked through the rulebook but could only find the sleep spell, but nothing on sleep potions or enchantments. I know its going to be up to the DM anyways, but I was curious if there is a "legit" way to have sleep darts.

Yes! We introduce sleep-poison in Lairs & Encounters.

A single sleep poison can treat twenty missiles (arrows, bolts, or darts) or one melee weapon. When hit (or ingested), the target must make a successful saving throw versus Poison or fall asleep 2d4 turns. Each dose costs 500gp (25gp per arrow/dart).

 

Awesome! So if i get darts and this poison, and treat them with it, does it ever wear off if they are just sitting in my bag? Or is it something that i have to apply and then shoot within a short period of time?

From ACKS (in the GM's section) - 

Once applied, poison evaporates quickly, diminishing its effectiveness. On the first day, it will do full damage, on the second day half damage, and by the third it will be gone. Partially evaporated deadly poisons allow the victim a +2 bonus on his saving throw after the first day, and +4 after the second. Each hit with a melee weapon is equivalent to a day’s evaporation, e.g. the poison will do half damage on its second hit and then be gone.

Using poisoned weapons is not without risk. Whenever a character’s attack throw with a poisoned weapon is an unmodified 1, he has accidentally pricked himself. He must immediately save versus Poison or suffer its effects.