See also: ADS-B Test | ADS-B Out Overview | ADS-B Portable Transponder
We tested the UAvionix prototype and it worked. Unfortunately, the market size and regulatory environment is too uncertain, so they won’t be marketing to us. You must have an N-Number. We appreciate their effort and hopefully it eventually works out.
It’s possible to get an N-Number, but is a hassle and may not last once they figure out that the system wasn’t designed for us. Mark Ingman and Harley Milne have each got one, but it took several FAA interactions. Harley flew a cross-country to Oshkosh, WI, with ADS-B out and it tracked him quite well as shown below. He is VERY active flying cross country which made the hassle more worthwhile.
There are other possibilities, like buying one made for a drone, but then you’re more on your own. Airplanes will still see you, but you’ll show up to them as a drone.
So for now, there aren’t any good choices. Stay visible, avoid where aircraft congregate, and keep a good watch!