That's a tough one. I know you can buy the 'Sever Magicka' spell from Sulinus Vassinus in the Skingrad Mage's Guild. Being a Restoration spell, I would imagine that 'Fortify Magicka on target', if it can be bought at all, might be available either in Anvil or in one of the Temples. If you don't want to search all over Cyrodiil for one (and who does?), just leave the cave and wait three full days (72hrs). The scrolls in the chest will re-spawn. Hope this helps.
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