The game files carry drop rates, but they don't match what players see, so Kinah Ledger doesn't print them. As an example for a Common item at 1 in 25 kills, half of players would have one after 17 kills and 9 in 10 after 57. Your server's real rate fills in as players submit sessions with the tracker.
Other places Lesser Soulstone comes from
Bosses, field bosses, named elites, dungeons, raids and events. These aren't open-world farming: Field bossDungeon
In the open world, the best spot is Contaminated Marsh in Chaotic Middle Reshanta, from Abyssal Mutated Ancient Destroyer, Abyss Drakan Strike Force Captain and Abyss Nagarant Brawler. It also drops in Corroded Heart, Distorted Old Growth Forest and Steel Titan's Grave and more zones. Bosses and instances drop it too, including Enraged Garzykan Swordsman and Enraged Garzykan Guardian.
Can you farm Lesser Soulstone in the open world?
Yes. Regular monsters with spawn points drop it in 21 zones, some of them in the Abyss (PvP).
What is the drop rate of Lesser Soulstone?
Kinah Ledger doesn't print the game-file rate because it doesn't match what players see. Real rates per server fill in as players submit sessions with the free Kinah Ledger tracker.
Kinah Ledger