Read about Blackwater (or "Xe" now) for some recent RL inspiration
I'm half kidding, but really, their story is amazing.
---edit---
I don't mean to imply they are "assassins" - but rather their story is very interesting and has aspects that translate well to piecing together a secretive organization, such as an assassins guild![]()
I have the Den of Thieves... I'll look for the Canting Crew.. that might be a harder find... I have Dragon #64!Unless you see an assassins guild a lot different from a thieves Guuild then Den of Thieves for 2e and Canting Crew a 3pp for 3e should be very helpful.