Where can I get them? Are they only in colossal chests and that another expensive ones? Or is it possible to get them from other chests or gauntlet?
They’re from the store chests (Massive and Colossal). Also once you’ve unlocked them, i guess you can get them from any kind of chest like multiplayer (but with a low probability).
I got one, free, from an Expand Your Army quest.
Also seen them for sale in store - the page you can spend alliance tokens on. But, in the first box for diamonds. It was about 1300 diamonds for a seeker missile