They usually drop one empty and one filled gem. Every boss will reward you with a gem after you kill him. If you don’t want to spend your money, visit different public dungeons and world bosses. The cost of empty gems is not high, while full usually cost a lot. The easiest way is to visit Mages Guild and purchase them there. There are different weapons to obtain gems. Crown - Trap any level soul and fully charge weapon of any level.Here is full list of Soul Gems in Elder Scrolls Online:
