The bfg features as part of the storyline, you even get a cutscene showing you mercs using it against zombies. After that you get them thrown at you in abundance. Just because you've never heard of it doesn't make it rare. Likewise, the bingo ball might be in plain view in a location you get an optional rescue at, but thats the only one of them anywhere in the game. That makes it rare, not hard to find, but still rare.