Honesty, you really don't need any moss if you use a good substrate. I usually use "Forest Floor" substrate by Zoo med. It holds humidity very well and when I see that my guys are coming into shed, I will give the enclosure a spray with water and they have wonderful sheds everytime. Another option is a mix of eco-earth and repti-bark. It is a little drier but when you wet it down it keeps humidity just fine. Actually, one of my guys just shed using that mixture and it was a success.
DON'T get caught up in spending your money on unnecessary products. It's easy to do, but in the long run alot of these things are not needed for BP's as they are easy keepers. :)