Fluorite is a great material to collect in Kingdom Hearts, since it it needed to upgrade your Keyblades. You need to use 1 x Fluorite to level up your Keyblades from level 0 to level 3, so you need to have a total of 3x Fluorite per Keyblade you want to level up.
Since there are 16 different Keyblades in the game, you will need 48 Flourite in total. So let's learn how to get Fluorite in Kingdom Hearts 3, so that you can upgrade all your Keyblades without any issues.
There are currently three consistent methods to get Fluoride in Kingdom Hearts 3:
1) Buy it from the Moogle Shop for 500 Munny per Fluorite.
If you got some spare money, then the easiest way to get extra Flourite is to just buy it from any Moogle Shop in the game. Each Fluorite cost 500 Munny, so it adds up if you need a lot of it, but it's definitely not a too high price.
They are not avaliable to purchase until you have beaten a few worlds in the game, so you can't do this right away, but it's a great option if you come across a new Keyblade later in the game and want to get some more Fluorite easily.
2) Shoot asteroids in space with your Gummi Ship.
When you are flying in space with your Gummi Ship, asteroids will occasionally appear. Shoot these down, and they have a chance to drop a Fluoridt. This is a great method for farming them, but it takes a good while to get all 48 you need.
3) Search Treasure Chests in Olympus.
After have you cleared the Twilight Town, you can return to Olympus to open all the Treasure chests there. Many of these have a very high chance of containing a Fluorite, so this is a quick and easy way to farming them.