I am not lucky, I just bought them and trust me - for such small models you do not really need a lot of pieces! Especially if you reuse them. Build something, photo it, let it gather some dust for week or two... dismantle and reuse pieces for something new.
The dragonfly was built out of all the purple elements that have accumulated over years of buying sets. This is a rare color and even in large sets if there are any purple pieces, there is just a handful of them. So I had... few of those tiny triangle pieces, few of those bigger ones with yellow ends, and some "armor" pieces from one of these ugly old kingdom sets. You know, action figures out of lego. Ugh.
So it is not that I had a big bin of purple pieces and thought "hmm, lets make a dragonfly and make it PURPLE, mwahhahahHAAHAHA", it was rather "heh, I have that dozen of purple pieces... let's make ANYTHING from them if possible".