Besides field boss and world boss what else drops this item? Do boss scrolls drop them? I got my answer from wiki, and it says yes   they drop from boss scrolls.

just got one from urugon scroll

The wiki page for artifacts lists where each of the various artifacts and lightstones drop. In the case of the iridescent lightstones, it says:

[How/Where to Get Irescent Lightstones]

 - Mountain of Eternal Winter: Jade Starlight Forest, Winter Tree Fossil, Murrowak's Labyrinth, Erethea’s Limbo Reward, Erebjork

 - World Bosses: Quint, Muraka, Kzarka, Nouver, Karanda, Ancient Kutum, Offin, Nightmarish Kzarka, Bloodstorm Nouver, Stormbringer Karanda, Thundercloud Kutum, Garmoth

 - Field Bosses: Katzvariak, Black Shadow, Dastard Bheg, Red Nose, Dim Tree Spirit, Giant Mudster

 - Guild Raids: Giant Mudster, Ferr, Puturum, Khan

 - Dark Rift: All

 - Boss Summon Scroll (Normal / Awakened / Darkened): Red Nose, Giath, Dastard Bheg, Muskan, Dim Tree Spirit, Ahib Griffon, Urugon

So, basically, right now, all you can get them from is the various bosses (though not all can drop them), but once the winter region is released, you'll be able to grind for them there. Either way, given how critical they are for the best lightstone combos, odds are that the drop rate will be horrible.