If you are buying a new camper they likely will supply one. Mine came with one in the kit. Hardware and variety stores typically have a small rv section, and these fittings by Camco are typically in those sections, as are water pressure regulators and right angle hose fittings.
You don’t need much pressure to blow out the system, so I have been connecting mine with a short length of surgical tubing to act as a pressure limiter by blowing off the fitting if the pressure builds up. I do this for the same reason they tell you to use a water pressure regulator.