You can find 'Backpacks' from loot spots, crates, or Supply drops . The 'Backpacks' range in size by small(9 slots), medium(18 Slots), and large(27 Slots).
You can equip your backpack by pressing B by default to access your CD:A Inventory and placing you backpack into the Middle slot on the left column.
There is also a 'Gun Bag' that works is the same as a 'Backpack' however you can only place guns and ammo in it. Unlike the 'Backpack' which you cannot place guns in. The 'Gun Bag' can hold up to 18 guns or ammo.