Hey :) For the death animation, I'm working on a "extra-animation" asset pack because a lot of people asked me about more animations. But for the "casting spell" one, that may be something too specific so I recommend you to simply contact a freelance animator and ask her/him to work on additional animations for your game :)
And for the spritesheet, here is a tool that I usually use to make spritesheets: https://www.codeandweb.com/free-sprite-sheet-packer It's very useful and it can adapt for your specific needs !