The DLC's have legendaries. The IOA (Isle of Armor) has Kubfu, which you can evolve Urshifu, and the Crown Tundra has the Galarian Birds, Swords of Justice, the Regis, Calyrex and the horse of your choice, a gift Cosmog, and a bunch of other legendaries from Dynamic Adventures or whatever it's called. However, the DLCs cost money, so if you aren't willing to spend some cash, I'm pretty sure there's no way to get more legends in the base game besides trading.