This is something i have pondered a lot about over the years for in most cases I find that the source of magic is not developed enough.
I liked the story of Materia in FFVII, where the "Life Stream" of the planet is condensed. An idea I have played around with is "True Knowledge" magic where those that truly understand the rules that govern there world can manipulate there world in different ways i.e. if someone truly knows why and how gravity works they can change the direction or strength of gravity in there immediate surroundings. In this system mages are more like scientists than classic mages.