Hi Ben,
The easiest way i have found to grow moss is to go looking for it on foot paths, tree stumps and anywhere that tends to get a lot of water. I just scrape it up and take it home. Best find was an old derelict house i found, the BBQ area was covered in it.
I have read a couple posts where people have found moss and ground it up at home in a mortar and pestle and then kept it in a zip-lock bag in a cupboard for when they needed it. When they were ready to have moss on their bonsai they would sprinkle it on the desired areas and mist spray regularly.
Another thing i have seen is to collect moss and then keep in in a container with a little bit of soil. The moss will grow (slowly) but at least is is always there when you need it.
The main thing to be careful of with moss is that you dont get ones that are to thick and are actually going to stop water from getting into the soil. Also watch out for weeds in the moss. I had one case where i had collected all this lovely moss and after about 2-3 months in spring grass had sprouted though out the hole pot and was starting to interfere with the bonsai roots.
I dont tend to bother with moss anymore as non of my bonsai are good enough to show and the moss tends to add more work to the limited time i have.
Hope this helps.
Giles
