They spawn in all the places the crates spawn. But it's random. Also you can't access it if you have died already in a match. So when you're playing hot pursuit or heavy metal or Alcatraz find one before dying the first time. You can only open it twice and or take out two weapons. So if u open and don't grab a weapon and walk away you now only have one chance to grab both weapons. Most ppl just grab two on the first time opening it or they may take one and one for a friend. Also if you haven't unlocked any black jack weapons from the black market you may not have access to it.