Magica is everywhere. It suffices the air, the water, the ground. Passively it enters our body, although if you train you can draw it in more quickly. Humans are decent magica vessels. However some monsters, beasts, and even plants can store far greater amounts. But the absolute greatest magica sponges are Demons. They have basically no upper limit, and can even compress the density of their body to further condense the storage of magica within them.
This is why they make the perfect magica batteries. Of course they are extremely hard to capture in one piece, but that's not a huge problem as even cut into pieces each part will radiate magica for years or even decades.
They are the perfect clean, constant and reliable power source.
There is the small downside of having to ignore their attempts at pleading and bargaining with you, but mental domination magic can solve that as well.
-Writings on the successful running of a Walled City by Queen Augustine