You can get Sunstone from a number of sources, unfortunately, there is no way to guarantee that you'll be swimming in them.
1. A number of bosses drop Sunstone. Most of these are located in Azteca, but a few Celestia bosses will drop them. The bosses of the "Trial of the Spheres" instance are probably your best bet.
2. Sunstone can be harvested as a rare drop from Sandstone. Sandstone is most common in the Savannah zone of Zafaria.
3. Sunstone can drop as a reagent harvest from Maelstrom Snap Dragon and a few other plants.If you plan on crafting gear and stuff after your crafting quests, I'd recommend taking up gardening, as there are a number of fairly common plants dedicated to dropping useful reagent, such as Maelstrom Snap Dragon (Celestia reagents), Burning Snap Dragon (Fossils and Lava Lillies occasionally drop), and Pink Dandelion (which drops gems and vials- even though this is a Crowns item, it drops from a lot of bosses)
Best of luck!
- Talon Shadowstone, Level 77 - John Forge, Level 49 - Talon Icewraith, Level 18
Do what I'm doing. Hoarding and bazaar Lol, I go around science center and look for sandstone for transmutes and chance of sunstone. I then transmute ..... And stop at the bazaar early in the morning lol to look for sunstone and sandstone. May take a week, but very rewarding !! OH WAIT!!! I almost forgot ..... I fed some unwanted cl gear to my pet , and got 2 or 3 sunstone! Try to feed cl gear you don't want to your pet instead of selling at bazaar. Useful technique !
Yes the archivist sells polymorph treant and king parsely drops the sunstone and drops about 3 to four seeds so you don't have to farm bosses that drop them long to get enough (they make good pet snacks after you have your garden overflowing with them too :)