I think you can find them on any of the tool trucks Such as Mac, Snap On, and others. NAPA Auto parts house should have them, maybe even Harbor Freight. Just be sure the adapter fitting to your air source is compatible, many of them look similar. Generally there is a rubber "O" ring at the spark plug hole. Be certain that the threaded part that screws into the head is no longer than the "reach" of your spark plugs. The reason for putting the vehicle in gear, and setting the parking brake is to prevent the crank from turning when the air pressure is applied. Keep the air pressure applied while working on the car. It will be normal to hear some minor leakage past the rings, which will be audible through the
oil breather.