On Rebecca's Garden on HGTV (October 2, 2009), she had a segment visiting someone's garden, and the owner had a 65' waterfall up the hill in his very big yard in Tennessee. Even though the yard was only about 6 years old, his rocks were covered with gorgeous, thick moss.
He said this is how he does it: He has a "secret" rock out in the woods somewhere, that has a lot of moss on it. He will get some of that moss, removed from the rock in a sheet, and put the moss onto one of his own rocks in his waterfall. He says you must do this after a HEAVY rain. He uses bird netting to invisibly hold the moss to the new rock, until it grows roots and grabs on for itself - and to keep squirrels, etc. from digging under it in search of worms.
He didn't say anything about buttermilk or yogurt or any other product to get the moss to start growing. He simply said get the "seed" moss, attach it to the rock, and then keep it damp.