I've had pretty bad luck sprouting sphagnum from the dried moss. I have used both Mosser Lee and the Chilean Orchid Moss.
All that ever happens on my pots is that the top layer begins to turn black:
Then, towards the end of the growing season, some unknown Bryophyte sprouts. It looks nice, so I leave it. The ambient humidity and overall sunlight is probably unideal. However, I think getting live sphagnum wouldn't be so bad.....
I also tried sprouting it in an enclosed container, but without any luck either. All I got was mold/fungus.