Magic in my world is tied to elements and their existence in the local environment, so much that people who use it are more of a conduit able to control it than anything else. But the local environment is very important. So if you're in the middle of a desert and you want to use magic dealing with water - well you're out of luck. Mostly. You could dessicate yourself or a companion. But that would be eeeeevvul!

That's the main source of magic anyway.