It seems like the problem is, you need to perceive the source, and then be able to affect it and channel energy from it. For me, perceiving the energy around me is easy; my eyes let me do it naturally. I don't know how other mages do it. And when I replicate a spell, I use that energy, even if the mage who cast it originally used something else. So, you need something that you can perceive and draw from.