To get Astral Crystal Grain in tales of Arise, you will need to loot it from blue chests in the game. Certain chests will often reward the material, which can then be used to make weapons such as the Basic Rifle and the Crude Sword.
If you want to find Astral Crystal Grain early on, the best place to do it is while completing the Glanymede Castle portion of the game. This point will be when you are fighting the first lord in Tales of Arise.
However, if you already completed this section of the game, you can always return if you haven't got the Astral Crystals from this area yet.
Nonetheless, on the castle's fourth floor, you can find the Astral Crystal Grains inside a blue chest in the room shown below.
Once you loot the chest, you will get the AStral Crystal Grains you need to make both the Basic Rifle and Crude Sword back at the Blacksmith NPC in Ulzbeck.
You can also craft both weapons if you progress enough into the game by visiting a camp. There will usually be an NPC merchant at camps, and by speaking them, you will also get the option to craft weapons.
Check out some of our other Tales of Arise guides!