I have completed the Master Destruction Quest. I got the fire spell upon completion which is a 100 point fire damage spell. I returned to Faralda to buy the frost and shock spells. She had a master frost spell which does 75 points but no shock spell. Can I get the missing spell elsewhere? Also, why the difference in damage output by the frost as opposed to the fire?

Do you have the Augmented Fire Perk higher than your Augmented Frost?