Collect moss from areas where it grows naturally in sunlight. The types of moss that develop in different conditions are often different species.
I find a nice variety of moss in fairly bright sunlight growing in parking areas and believe it or not - in the cracks of sidewalk - where it is subject to frequent irrigation.
You can crumble the moss onto the surface of your tree or try to plant it in flats. Don't cover more than 50% of the soil surface with moss, and even that much is pushing it. It interferes with diffusion of air into the soil.
I think you will find more articles on moss if you search in
bonsaiTALK Links under "MOSS" or look in the Repotting->Techniques folder.
TreeBay Bonsai Tools & Supplies sells moss spores which can be used to innoculate the soil with a nice variety of Japanese moss. That is another option.
Regards,
Matt