I, personally, haven't seen any. Royal Thorns are easier to "sprout" in the bounty hunter ot klisan's shop. I also have a feeling that those thorns will do better against the Kraken rather than K'Tahu.
You should try the kraken as the next boss battle. I remember beating him up rather well with only 4 royal thorns (and did that before K'Tahu). I also didn't have stone skin and used magic spring exclusively (had to beat the Kraken to get stone skin, as it was in the human lands after the troop upgrade, but the 4-battes quest was already impossible since I found the 4 thorns in the dwarven lands, nowhere else). You can use the following tactic: hide the thorns between 2 tentacles and start using flaming arrow to drop all of them down to 1-hit. Then simply drop them down one by one, hide the thorns in a corner so that they take damage from only 2 stacks of devilfish at the time, and with the ever-running magic spring you'll never run out of mana.
With more than 4 thorns the battle will be much faster, and you will do well to avoid killing any tentacles before the time is right to do so. If you kill a side tentacle and hide in that corner, you will still take damage from the kraken (posted that already, rancor did as well).
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