The Lightning Knight costume is exclusive to the original browser/Flash version of Poptropica (Super Power Island). It does not exist in the DS cartridge game Poptropica Adventures (2011).
Complete islands (Early Poptropica, Shark Tooth, 24 Carrot, Time Tangled, Super Power, etc.) for DS-exclusive outfits: Golden Crown, Explorer Gear, Super Villain Suit, etc.
Play the original web version via Ruffle emulator (browser) or the Poptropica Steam app and complete Super Power Island.