I have gotten 4 event coins and haven't spent a cent in this game. Used 2 on egg smash thing and the thing with pumpkin duke near it. I know I got them and I don't remember which event I got it from. I also asked live support and they said it was impossible
Those items are really rare. You can't get them unless IGG decide to give to players for free from events, giveaways, free packs etc.
If you don't want to use cash in game you need to check all events in game, all offers in bazaar, cause you will never know when IGG will give it to everyone.
Unfortunately there is no other way.