Nova crystals are simply the g-class version of light crystals. You can get them by killing a kirin, smashing a gycperous' head crest, mining them occasionally and repelling Black or Crimson Fatalis; all of these are at g-class remeber. I personally repel the black fatalis because you get a lot of money (36,000z) and multiple nova crystals as well as fatalis parts, but it would be easier to smash a gycperous, regular or purple, head crest with multiple pellet shots. If you're going for the gycperous quest, I'd recommend the dual quest at HR4.