Stealth Boy

 in Horizon is a consumable and crafting ingredient, invaluable for crafting Personal Cargo Bots upgrade modules in the Tech Lab.

General

 * Settler Missions can yield to a Stealth Boy
 * a Nightkin can sometimes carry an intact Stealth Boy

Traders

 * 1x Tinker Tom (Railroad HQ)
 * 1x Myrna (DC)
 * 1x Cricket (Caravan Vendor)

South Boston Military Checkpoint / Andrew Station (2)
NOTE: Andrew Station (Blue Trailer) is one of the most defense ways to get 2 Stealth Boys without any encounter to fear. You simply need the "Wicked Shipping" trailer key and your done!
 * head to the West of the station OR
 * to north of the Checkpoint and
 * unlock the blue trailer with the Wicked Shipping key

"The Big Dig"

 * inside Hancock's stockpile (3)