First decide what manufacture you want, Blue Ox, Roadmaster, Demco are the main ones. Personally I prefer Blue Ox, great product, customer support & ease of operation - hook-up & disconnect. Another major plus IMO is when it not being towed it doesn't look like a toad.
Then find a dealer in your area, either on the company's web site or better yet call them up and ask for the rep in your area, he or she will know all the dealers in the area and which one to go to and which ones to stay away from. Personally I would stay away from almost any body shop, installing baseplates aren't difficult but you have to be understanding that the instructions may or may not be accurate.