Are royal thorns actually recruitable ?
 

I restarted 20~30 times hoping that the inn (near the kings brother) that can give a random level 4 monster, would give me royal thorns. It never did.

I suspect royal thorns have been removed from the game in the later patches? Or is there a specific place that they *sometimes spawn*?

A place I found them in was Verlon Forest, at the hut relatively near the Ghost quest. (Not the one to the right after emerging from the portal from Greenwort)

I saw (rarely) few of them in green frog castle. I also saw them rarely in pirates islands. Often you find many in Elf islands. But you don't mention the game version you use, mine is retail 1.7.

Found them in the swamp off bagard. 1.7

they appear sometimes in frogus bagagabaga castle but in low numbers ...

They are also in very low numbers in any Darion area or Pirates Islands. I only remember to have seen good stacks in Elf islands and not in all games.