1) Find familiar can only be cast by the "protaganist"--you. So if your character isn't a mage or sorcerer, you can't cast it. I have no idea why they did chose to implement it that way, but I guess we're stuck with it
2) You would kill the good silver dragon?!? You must be a very evil person indeed!
Just keep advancing in the main quest line (i.e. head for Spellhold). You can't miss her.
Hope she slaughters that evil party of yours...