Ideally when landscaping with any plants, including moss, you will want to find a local retailer that is selling native species. Those are species that naturally occur in your area. These plants have not been introduced purposefully or accidentally by humans into the wild. Not that there are any aggressive invasive moss species. (I don't know of any invasive mosses off the top of my head, but I will look into it more.) Needless to say, you do not want to be the first person to release the moss version of purple loostrife or kudzu into the wild, which are both invasive species in North America. It is also important that retailers you might buy moss from are growing it themselves. Removing large patches of moss from the wild and then selling them is bad. They should have actively growing populations of mosses that they are propagating to sell.
If you live in Connecticut, or in the surrounding small states, I have heard of a place to find mosses to use in the garden. The retailer is Sticks and Stones Farm in Newtown, Connecticut. I have yet to visit personally, but it comes highly recommended. They sell seven different species of moss that can be ordered online. Moss is grown in flats outside and they do not poach from the wild, which gets a two thumbs up from me. If anyone has visited the farm I would love to hear your thoughts or comments on your experience. I will definitely let you all know if I have a chance to visit.