Buy them from fences, general goods merchants, and some thieves guild contacts
Pickpocket them from NPCs who carry them
Loot them from bandits, marauders, and other humanoid enemies
Search containers in caves, ruins, forts, and dungeons
Check chests, desks, barrels, and sacks in town and wilderness locations
Complete Thieves Guild quests and reward sources
Visit the Imperial City Waterfront and other criminal hubs for vendors and loot
Use the Skeleton Key from the Nocturnal quest if available
Find them in loot drops while exploring Oblivion Gates and related areas