Citrine is available to mine from rock clusters in the Glade of Trust or Sunlit Plateau Biomes.
The Glade of Trust Biome will cost 5,000 Dreamlight to unlock and can be assessed via the Sunlit Plateau, Dazzle Beach or Peaceful Meadow biomes. The easiest way to access it is via the Peaceful Meadow biome, which is the second you unlock following The Plaza.
Alternatively, the Sunlit Plateau Biome costs 7,000 Dreamlight to unlock via the Peaceful Meadow entrance.