This depends on who you ask, as everyone has their own beliefs. It's hard to define magic, but I personally believe that the subconscious is what does the work, which I guess you could say would be your higher self.
Many practicioners also worship various deities, and one could say they are channeling them for power.
Also, many people channel the elements, whether it be the traditional four, or five elements for power.
It's all about perspective and your personal path. For me, I draw energy and power from the elements and my chosen deities, but it is my subconscious that does the work through visualization and concentration.
I would like to note that I definitely did not cover everything, since many practicioners follow, worship, draw energy from many different entities, beings, and much more. I also do not speak for everyone, as what I wrote is based on my personal beliefs and my own experiences.