Baba yaga, I could farm her easily but I just wanna help people in high level worlds. Basically, I have a short attention span when I have less determination heh.
I haven't seen a regular spell proof, but I do have a lot of school proof jewels (especially Myth right now). I've gotten them through regular questing at the weirdest times. I couldn't say for sure who has them, as I don't check until I'm pretty much done for the day.
Again, I apologize for the late reply. Currently, there is no known drops for a jewel that can be socketed to a pet that give the talent 'Spell-Proof'. However, if you want the Defy Opal (Give the talent 'Spell-Defy'), It has been confirmed that Baba Yaga drops it.