I believe you need to attack Level 7 forests or above with a great or elemental dragon on the march, and Level 8 or above plains for the helio dragon. They are very rare which is why I bought both of mine for 10 rubies when they were on sale. And in the past few events I've gotten Ultimate armor and scales for almost all of my dragons which automatically gives you the steel armor for them and allows you to use them in battle without farming for armor pieces. Cheers ^~^