99% of all gas fittings at the meter use this fitting to cap off the flow of gas to the underground line. It is available at any hardware stores.
Metal Flared Tube Fittings; Type: Flare Cap; Flared Degrees: 45.000; End Connections: Flare; Tube Outside Diameter: 3/8; Material: Lead Free Brass; Max Working Pressure Dynamic: 1,000
Tube Outside Diameter
Max Working Pressure Dynamic
Max Working Pressure Static