Screw a 1/2" pipe coupling on the pipe and use a 1/2" MPT x 3/8" half union in brass. Or, use a 1/2" x 3/8" black pipe reducer and a 3/8" MPT x 3/8" half union. Maybe Lowes carry those fittings. The fittings aren't exotic. If all else fails, buy a gas whip kit. They usually have the FPT by flare adapter in the kit. The McMasters # for a fitting that will screw directly on the pipe is 50635K441.