Element magic in the DV doesn't necessarily require you have a source of that element. Harry can just conjure fire "out of thin air" - though he can also use fire magic to move existing heat around (freezing the lake surface in WN by creating fire).
Also, a lot of the elemental magic doesn't necessarily involve manipulating the physical 'stuff' at all. Carlos uses water magic to shoot green beams of disintegration. Harry uses earth magic to crush things with massive gravity, and according to YS you can use earth magic for lightning (electromagnetism) too. Elaine's hyperawareness defense may be air magic (YS lists it as "spirit or air"; air is tied to knowledge in DV, that's why Bob's called an "air spirit" as well as a spirit of intellect).