I personally think that the evolution stones are too rare to get from poke stops. I think that a chance to get a stone from a poke stop is near impossible or my luck is just bad... Another choice is to find the mystery man but that's also very hard since he changes places every five minutes.
I have 5 of each stone... pay attention to Discord. They announce when mystery man spawns. It's quite easy to get them.
I've also been lucky enough to meet the Mystery Man three times! I don't think there needs to be too many evolution stones in the game. It's a great coin sink.
