Levels
Lv5 and Lv5+ don't drop. Clear a Nova Crest Palace stage with a Lv4 or Lv4+ crest on your leader unit, and it can evolve into one (Crest Burst).
Obtained from
Crest Fragments
A crest you don't apply to your leader unit converts into Crest Fragments of its level. Ten fragments combine into a crest of that level, or convert into one fragment of the next.