You can get new weapons from the following sources:
Weapons are randomly generated from these sources. You can increase the chances of getting higher ranked weapons by upgrading each weapon type’s Mastery skills. The weapons you unlocked are recorded but you don’t get to keep them permanently. Whenever you restart a story or switch game modes, you’ll start off with the default, low-ranked weapon. The Custom Loadout game mode gives you the freedom to select any weapon and ultimate spell you have unlocked so far.
If you find a good weapon (good attack power, spell, and passive effects), hold on to it until you find a better one. You don’t need to switch weapons often unless you find a better weapon from the one you currently have.