There are also a few style materials that drop so rarely they are effectively decon only. Or, they are one of those “gather 10 to make one style stone” deals. Dwemer frames are way easier to get by doing Darkshade and deconing Automaton, for example. Similar story with the Great House motifs in Morrowind.
Plus, Skinchanger and Hollowjack will drop again during their festivals upcoming at the end of the year. I see in the PTS patch notes that they've reduced the droprate on Amber Marbles (style mat for Hollowjack) for the Witches Festival. But, you'll still get a ton of it if you participate in the festival. Same with Wolfsbane for Skinchanger - you'll get a lot of it during the New Life Festival at the end of the year.
Really, it's just the Crown Store exclusive motifs that require mimic stones (though you can decon gear crafted in that style for a chance at the style stone, e.g. Stahlrim for Frostcaster motif). But, these motifs are never required for writs, and unless you actually want to craft something in one of those styles for yourself or a friend, you can just use them in an outfit.
Thanks for the replies! I'm asking because I don't want to waste my time looking for something I'm never going to find in the wild. I guess I shouldn't have expected any less from the crown store. Of course it requires another purchasable item in order to craft something for a friend. Lame.
So there's Skinchanger, Hollowjack, Worm Cult, Harlequin, Frostcaster and Tsaesci. Got it.