From Inventory in The Hub:
Fey Crystal
Found: at ponds and wells (after getting the 1st shard), by ****ing skunkupines & fangbeasts (after getting the 2nd shard) or harvesting wheat, strawberries, cocoa beans, sugar cane & lucky clover crops (after getting the Agricultural Tabernum shard), Faerie respite, Sir Pigglesworth, Goddess tree, Plume hive adventure, Faerie bridge, Glimmer Fountain,
Uses: item in faerie essence
Best places I've found are the Faerie Bridge & Faerie Respite.