The Swallow Moth cannot be bought; it must be found. There are empty Swallows at Highrise Construction (Highrise), Waterfront Booze (Riverside), Riverside Construction (Riverside), and Alpha Construction (Alpha). Note: Swallows only appear in v2.01 and higher of the game.
Enter the "Controls" option at the game launch screen and reconfigure the joystick, then choose any button and change its value to "God Hanger". Start the game and press the reconfigured button to reach a hanger that contains various weapons, items, and money. Note: Your Moth will be targeted when game play is resumed.