You can buy sphagnum moss from most garden centers and can order it off the internet (which is what I do). Just make sure you get one that is free from fertilizers. Usually the cheapest can be had from orchid supply companies, those that are marketed for pets usually have quite a price increase.
Although for simplicities sake, I'd just use aspen in there. Although, most of the aspen I put in my snakes' hides usually end up shoved out, so I don't even bother anymore. They seem to like the smooth bottomed stuff. Which is why they get under their hides and shove all of the bedding out of the way.
You don't cover the cage bottom completely in aspen? Why not? It's pretty easy for me to see when my snakes have pooped. I just drag my hand around in there until I feel a clump and remove it and small bit of what was around it. Pretty straightforward.
For rocks and smaller sticks (that can fit in a soup pot) I just boil them for 30 mins or so. For larger sticks/bark pieces, I bake them in a 150-200 degree oven for an hour or two. Although do make sure to keep a close eye so that edges don't start smoking if they're too close to the elements.
I wouldn't bother collecting moss from outside, the potential for bringing in bugs and such would be too much to fuss with, imho.
