The game team is aware that it can be hard to find the evolution shards you need in order to evolve your main characters. That being said, it is possible for evolution shards to drop anywhere, so just re-playing the hardest levels you're able to beat is a good way to get more loot and level up your non-max characters.We will be releasing a feature in the near future that will allow you to specifically hunt the evolution shards you need. Unfortunately I can't share more than that right now but we are excited to release this feature as soon as we can.