One of the main task for everyone in the Division is to get the best gear, modes and weapons. The top loot is marked with yellow colour and these drops occur rather rarely.
There is no place or way to guarantee the high end drops falling but you can influence on their frequency. You should to equip your character with Scavenging bonus gear that increases the chance of the top loot dropping.
To get better this perk bonus you should look through the gear characteristics and find the Scavenging bonus. The higher its total number, the greater chance of high end loot dropping (in the Dark Zone and beyond).