For a good selection of potting mixes and components like coir, perlite, vermiculite, shale, etc, try hydroponic stores. Their products might be a little more expensive but - well- you get what you pay for. Also try good local nurseries and ask to buy a bag of whatever they use for their own growing purposes.
Miracle Grow is the only potting mix my local Home Depot sells now. I used to buy ProMix there, but they haven't carried it for years now.
Karen
