Kill or loot enemies that use Destruction magic, especially Atronachs and high-level mages
Check alchemy shops and general merchants for Glow Dust
Search loot containers in mage-related locations, such as forts, ruins, and guild halls
Use the console command `player.additem 0001EBE8 1` on PC
Use the spellmaking/alchemy-related merchants’ inventories after waiting 48 hours for restock
Harvest it from Will-o-the-Wisps if available in your version/mod setup