They can be found only at Herowares:
A legendary ship will have 3 gems, it’ll look like this:
Because legendary items are better they are very rare. This topic tracks legendary items’ locations, check its posts to find out the location of the item you want(NOTE: The location changes every 4 weeks so don’t check the faraway posts).
Little changes like smaller hitboxes and more speed for Bombers(that’s all the information Ik).