Where to find royal thorns?

[Centipede]

I found 4 on the first island and now I am up to Verona but I can't find anymore. I've been using them forever even if they are less than 1/3 the max stack, since they seem better than almost all troops if I want to get no casualties with the summoned thorn spam.
Also, am I right in thinking that if I've achieved the grand strategy medal level 3 I don't have to worry anymore about casualties? That will make life a whole lot simpler.

[ZumZoom]

Units and their location is randomized when starting a new game (aside from always having human army units in the main castle). Royal thorns may seem like great units at first, but will become pretty useless as the game progresses.
Grand startegy medal is useful for that bonus you get, but it's pretty much irrelevant if you're playing on any difficuty below hard. I still try to not get losses regardless if I got the medal or not.