Unable to acquire new rare units

Thats cause of limit of some rare units. I cant remember to own more than 3 Legends at once. Walls, infinyte fortresses and sanctuary beacons seems to ve unlimited. Arcane wraths, legends, tesla and so go on is limited in numbers. I suppose it is what you trouble with. If not, may be you farm not enough hard missions. Thor and valerian orbs start to drop from mission with Nikkis mega-craft. And one more thing: build more tech centers and update them. Every one of them have skill that grow chance to drop rare unit. When they not needed you always can store them in bank or just scrap