You can find most weapon cards from Encounters, Infestations, Group rewards, as well as when you leave survivors in an Armory. The only weapon that's different are the Sniper Rifles. You can only find those from completing Raider missions or Seasonal Group Boards.

Once you have unlocked a Sniper Rifle by finding it from the rewards in a Raider mission you will then be able to find them from Packs.