This is how to obtain the Andromada from the mission Stowaway:
Before starting this mission, it is reccommended to date Katie Zhan, as you will keep your weapons and get out of hospital for free if you date her. She can initially be found at the northeast corner of Avispa Country Club at the south end of the city, practicing martial arts behind some bushes. She can be seen easily from the road. Katie likes men with sex appeal and muscle, so bulk up before asking her out.
Get a Rhino and store it in the Verdant Meadows garage. Once that is done, get a minigun and start the mission. Wait for the mission cutscenes to finish (although you can skip them to save the wait).
Once the cutscenes have finished, destroy the Andromada with the minigun. Then kill the secret agents, open the garage door, block it with the PCJ-600 (by parking it against the door), get into the Rhino and use it to push the Andromada into the garage. Once you have done so, waste yourself. You will fail the mission, and you will lose all of your weapons (unless you are dating Katie Zhan), however, if you are lucky, the Andromada will be stored in the garage and will be ready for you to pilot anytime.