What Dr Strangelove is trying to tell you is that, in Cantha, Greens can drop almost anywhere from any number of bosses. Factions is still so new that I doubt that there's even a complete list of greens yet!
Go out there, kill bosses, get greens!
If, on the other hand, you're talking about Tyria, then it's a totally different matter. Tyrian greens only drop in a few places from a few bosses. Either check the greens pricing guide
here on guru, or the full list on guildwiki.
Sorry for no link - stuck at work behind a restrictive firewall and Websense (netnanny for businesses!)