Here's the deal with sphagnum. There is a particular type of fungus that can grow on the live moss, and that fungus is bad news for humans.
The sphagnum moss sold by the major reptile companies is not alive and has been cleaned and/or sterilized, so no issue with the possibility of fungus. Plus it is sold bone dry.
The moss sold at garden centers probably has been cleaned as well, and plus it is not alive. I think the bag should say whether it has been cleaned or not. It should be bone dry as well. I have never heard of anyone having a problem with the garden store moss, but I have always been too paranoid to use it with critters. I always buy the reptile stuff.
I have never myself seen fungus grow on sphagnum as used for humidity chambers. Where I live the moss dries out pretty quickly anyway before I mist it again. It is advised that any moss that does have some fungus be thrown away, the container washed, and new moss put in.
I also am pretty sure that the "live" moss sold for frogs and such is not really moss. I think it might be a lichen.