Some of the betters spells can be purchased on both Freedom Isles, you just need the map from Kitano or whatever his name is (beach in Greenwort).
If you still dont have luck on some decent spells, you will need to stick to units that can summon like royal griffs, rune mages, etc. accompanied by some archers and spam frozen spikes until you find phoenix or a decent aoe (like fireball).
I'm in similiar situation, the luck factor is greatest for Soothsayer, but I like when its challenging and finding a new spell is like a christmas
