It is sold by weapons vendors and spawns on Raiders starting at level 30.
You can get ammo by crafting it at the chemistry station, buying it at vendors or looting it from Mr Handys.

• Automatron DLC
  

Please check out my other weapon mod (that i actually put effort into) the Pulsed Energy Rifle.
http://www.nexusmods.com/fallout4/mods/14932/?