Actually, you can find the Junkers bribe on Planet Manhattan.
I checked my FAQs and found one for vanilla bribes that lists 18 bases where Junkers can be bribed: Battleship Missouri, five Junkers bases (Beaumont Base, Kreuzberg Depot, Rochester Base, Trafalgar Base and Yanagi Depot), Bruchsal Base (Bundschuh), Buffalo Base (Liberty Rogues), Gas Miner Naha (GMG), Helgoland Station (ALG), five planets (Hamburg, Houston, Manhattan, New Berlin and New London), LPI Sugarland, Mainz Storage Facility (Republican Shipping) and Norfolk Shipyard.