In case anyone reading this is currently on Kerkvaya SI, DO NOT READ

I am trying to think of any way to drug a PC or NPC outside of ruling that the poisons involved just cause a lot of fatigue. I cannot find any special knockout poison, however. Is there something statted in any book or does it need to be homebrewed?

Poisons and drugs are something you can easily just handle narratively. Your players are not going to argue with a well set-up scene as long as you let them RP a bit during it.

If it's something to push the plot, then talk with the PC getting the juice ahead of time and ask them not to spill the beans. Offer them something to let you dope their character (e.g. a +5 to a future acquisition roll, applied after the die roll is always tempting). If it's something that involves PC death (or even something less, like compromising a PCs social standing) then it's best to get mechanical with the rules.