lthough not the only place, the best place to find rare and special Items are in Supply Drops and Supply Stashes.
Supply Stashes are hidden crates that can be found in multiple locations. Supply Drops are air-dropped crates.
Be aware that when interacting with a Supply Stash or Supply Drop, you are vulnerable to attacks and most likely exposed. Have teammates cover you while you scavenge for supplies or make sure you clear an area before you collect.