Moss mesh/wire

You could get plastic mesh at a crafts store like Michaels, but I personally like using rocks and hairnets.

You can also take a shower "puff"/"scrunchy" thing (that you put shower gel on and clean yourself with). YOu just cut the string that holds it all together and you get several feet of ideal moss netting.

Wow that's an excellent and inexpensive idea, Dave! Thanks! Thank you as well, houseofcards. I will be using moss on rocks, but this is actually to go between the rocks, as I want moss to dominate a mountain-ish thing I am making. I'd be cutting the mesh to fit between the rocks.

Well since I have two reliable sources of the meshy material, what about the hard wire, so that the moss remains in shape and well.. in place? Can I get this at a hardware store with some wire clippers?

Dave is right, you can use the crunchies as well. I've used both, I did find the hairnets easier to work with and you don't need something to tie it with, you just make a knot. The regular hairnets also are fairly invisible while the cruchies are much more visible for longer until the moss grows. If you want moss between rocks I have found it's still easier to use rocks between the rocks.