You can also put more than one source in the same world like having blood mages sacrifice blood for results and have other mages draw life from plants or whatever.

One of my favorite magic systems was the Full Metal Alchemist one where (spoilers?) essentially they were drawing on lives sacrificed in a parallel world.

I am also particularly fond of the one from Patrick Rothfuss' new series The Kingkiller Chronicle which really melds scientific principle with magic in a kind of seamless way. As well as having and dealing well with latently magical things like fairies and whatnot. So you kind of have two approaches that he managed to work together. I'm hard put to describe it correctly. Go read the books, they are great.