How to Get a Backpack Early
There are four different locations where you can find a Backpack quite early in the game. The first is northeast of the shelter in the ravine. You will find an abandoned car here, where the backpack along with other loot will be stashed in the trunk of the car.
The second location is slight to the north of the shelter, where another car is waiting for you. This car will be on the road that goes to Santa Maddalena. There might be some enemies here, so be prepared for a fight before you can check on the loot in the car.
The third location where you can get a backpack is in Dejado, where you have to go to the hardware store and look towards the house across on the other side. Enter this house and you will find the items inside along with a backpack.
The last location comes across when you are completing the Running Date Mission with Alice. It will be on the mountain trail where it can be in two locations close to each other. Check for the backpack either in the convenience store or in the abandoned car nearby.