If you intend on growing them in containers for the long term, I'd like to suggest that you visit the Container Forum to do some reading about potting mixes. Not that you have to follow the 'recipes' suggested in that forum, but you'll come away with a good understanding of what kinds of individual ingredients can provide the best characteristics. Oh, and what kinds of things to stay away from! With the incorporation of a few additional ingredients, you can even turn icky potting soil into something good for your camellias.

You are correct that Miracle Gro will not be recommended over there, and you'll learn why!

Here is a link that might be useful: C'mon...I'll take you to the Container Gardening Forum!