There are a few ways to get Celestial Infusion.
The first way is to buy it from the Auction House. You can find it under the Miscellaneous section. It is a good idea to check the Auction House often, as the price of Celestial Infusion can fluctuate.
The second way to get Celestial Infusion is to craft it yourself. The recipe is available from the Master Alchemist. You will need the following ingredients:
-1 Jug of Water
-1 Pouch of Salt
-1 Vial of Vinegar
-1 Bottle of Alcohol
-1 Pinch of Stardust
The third way to get Celestial Infusion is to find it in the world. It can be found in chests, dropped by mobs, or given as a quest reward.