Where did the Kyber crystals come from?

Kyber Crystals

Kyber crystals were Force-attuned crystals that grew rarely but throughout the galaxy, with some areas having a greater abundance than others. The crystals concentrated energy in a unique manner through the Force, resonating with it. The crystals exhibited a sort of "collective consciousness," verging on sentience, and could non-verbally communicate with both one another and living beings. Some theorized that the permutations in the kyber lattice were comparable to emotional shifts in a sentient being.

There do exist Synthetic Kyber Crystals, which were less stable.

Synthetic kyber crystals were artificial kyber crystals,1 Force-attuned crystals that were used by the Jedi and Sith in the construction of their lightsabers, that glowed with a green hue. Due to their synthetic nature, they were highly unstable and explosive, and possession of them was made illegal by the Galactic Empire.