Are there anywhere else to buy them?

I've got 13 of them now, and I can have up to 16. I've unlocked the four "starting" islands and I can't find anywhere else to buy them. Will I at least be able to buy more later?

I can't find the sacrifice spell. Any other way to get more?

They're my most important unit, and the more I level up, the more potential power I lose for not being able to fill that unit slot with units.

I went the first 4 islands with only 7 when I could store up to 16. They are still worth it just for the resurrection. I had mine split into a 3/4. Have them wait at the beginning of the turn and then use them to rush in on anything that has wasted its counterattack.