dwarven mine gives tons of Soul Gems
also if you get the barter enchantment you can also enchant stuff to lower your buying price. I think i buy empty lesser soul gems for only like 20 gold. I have so many soul gems now that it's starting to become a weight issue.
i'm getting my casting cost down low enough now that i can spam single cast fireballs almost non-stop and never run out of mana.
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