Thanks for the great ideas guys, keep 'em coming
The thing with the "magic as item" solutions is, I think it places too much power into the hands of the person possessing it, be it PC-s or the BBEG.
It's the raw primal essense of magic from which all other magic is but a shadow? What exactly do you expect?
And on the contrary, the container's purpose is to ensure that the stuff doesn't escape. It's a container. You'll note from my description that the containers are generally extremely dangerous. I adhere to the Necronomicon theory of powerful magic. When you play CoC, players aren't eager to try out the new magic item because it might suck their mind or soul out of their body or leave them a withered husk or turn them into a servitor of some dark mad god. So it is with D&D artifacts. If I was a God hiding magic from mortals, I'd make srue that the container was lethally trapped. If you could use it as item at all is probably only a side effect of the container leaking.
Did your read the 1st edition DMG's description of artifacts? Most of them had side effects so nasty that you hardly dared use them.
And what if the item is destroyed?
Very bad things. I'm thinking 40d6 damage over a 4000' radius, 20d6 damage in the 2000' beyond that, 10d6 damage in the 1000' beyond that, and 5d6 damage (save halves) in the 1000' beyond that. Additionally, all in the radius of effect must make a DC 30 save or suffer an effect related to the school of magic: death for necromancy, form instability (per a chaos beast) for transmutation, feeblemind and permenent suggestability for enchantment, etc. Additionally, 81 living spells are released in the radius of effect - 9 for each level of spell. Additionally, at least one God is now very upset.
Not that the containers would be easy to destroy. Generally it would require a divine or comparable being to achieve this.
The basic problem I have with your plan is it doesn't look like the gods did a very good job of hiding things if mortals can pretty easily obtain peices. If it was that easy, why haven't the worlds archmages brought back powerful magic long ago?
But, ok, if you want an ocean of the stuff out there for the taking:
Transmutation: It's distributed throughout the worlds water. To obtain some, you have to distill a lakes worth in a magical still, but it turns out that water deprived of its magic is sterile and can no longer sustain life.
Evocation: Can be obtained from the surface of the Sun.
Divination: It's hidden in the brains of all sentient species. The Mind Flayers distill it and use it to power their alien magic, and if you want a lot of it, that's the easiest place to find some.
Illusion: It's the substance that dreams or made of. You have to have make a dream catcher to wind them up like a duster gathering cobwebs. However, areas deprived of dreams become colorless and its people apathetic.
Conjuration: The stars were made out of it. You either have to steal a star or catch one as it is falling.
Enchantment: It was hidden in the Elemental planes. It's an addictive drug popular with the genii's. It can be obtained in the market's of the City of Brass or from the well guarded fire poppies directly.