I'm only on my first grow, but I did a lot of research and many, many people told me that MG can kill a plant. Some said they had several good grows then all of a sudden, wham. If you use it, do so at your own risk.
You can get it at just about any hardware store, not sure about nurseries. But I've found they usually have the best prices on just about anything. Bought a 3" or 4" bag of perlite for $18, should last me a half dozen grows (8 plants) or so, plus clones.
Health food store is the best place for the molasses.
I'd recommend FFOF for soil though. Not only don't you have to add nutes for the first month, but it comes with plenty of Mycorrhiza which you can read up on here
http://en.wikipedia.org/wiki/Mycorrhiza The stuff is great for building bigger and more complete root systems, it help the roots you have absorb and process more of everything, and it even helps speed up rooting new clones. Not only would I suggest using FFOF, I'd also recommend picking up something with Mycorrhiza and low nutes so you can mix it with the soil, use it in your watering schedule, and use it every time you transplant.
Oh, and FFOF comes with a pH of 7, so you don't have to worry about soil pH either.
For clones/seedlings you'll want to start with something that doesn't have soo many nutes, like FF Light Warrior or something. If you go to a nursery ask for the horticulturist (there's always one on duty) and ask what they use for seedlings.
If you decide to go organic, I'd suggest using a compost/soil mix. Again I'd ask the horticulturist for something that's hot but not too hot.