Actually, a few bosses during the special quests of "To Sorrow's Furnace" won't drop greens; a good example is Gulnar Irontoe of the Kilroy Stonekin quest. Check the
Guildwiki page for what bosses drop greens. Also, the chance that you'll get a green isn't that high; currently from the runs I've done there's been about a 4% drop rate for a single party member. About the only way you'll get a specific green is if you kill these bosses constantly.