Moss does not grow from seed, but from spores typically carried on the wind. Here in central Virginia, if one builds it (an environment conducive to growing mosses), they will come (on the wind or however). I suggest that you take small pieces of the moss that already grows in your yard and place it where you want more moss to grow. It will take some time and probably some weeding to keep the open ground clear, but eventually your moss and others that might "blow in" will take over.

If you're still wanting to do this, you can create your own. ;o)

Here is a link that might be useful: moss