Where to buy more powerful Destruction/Restoration spells?
I'm getting to the point where my novice spells aren't doing enough, even with the perks. Time to upgrade to the Apprentice level versions of Destruction (mainly fire) and Restoration (mainly Heal). Where can I learn better spells?
College of Winterhold. They have a master for each school of magic. Every one of them can train you, give you tips on that school of magic, and sell you spells and enchanted apparel to help with your magic. It's pretty much a mage's (or anyone who wants
to learn about magic) wonderland.
Once you hit 100 Skill in any school of magic be sure to approach the master of that school of magic at the college. There is a Ritual Quest you can go through to unlock more powerful spells (such as firestorm, blizzard, lightning storm, storm thrall, call
to arms, and guardian circle).