You need the sedative from that dragon guy that makes tea he will gjve you two for free.
Go fight a bug bear and either bring it to less than half total hp or tease it as high as you can without...you know.
Use a sedative on it and youll get a scene where it goes to sleep and you get the saliva.
If you used all your sedatives and still didn't get the spit or enough of it you can make it yourself. You et the receipe at the start of the quest can can make it at the sewing machine.
