There are a couple of locations in the game that will almost always have 3-star weapons and sometimes 4-star weapons, armor is common too. These locations are Factory, Harbor and crossroads. If your wondering where to look for the weapons, each of these locations has a large building with lots of weapons.

This is a mechanic that people might not know about yet but the health vending machines can also give you weapons. every time you hit a vending machine with a gun or melee attack two times it will spark which has a chance to drop a gun. I've noticed a pattern with the drops, vending machines will drop a 1-star/2-star gun once and a 4-star/5-star gun once and one ammo pack, after that it seems to drop no more weapons. I just want to emphasize that this method is very slow and you should find a vending machine after leaving spawn if you want to make it in time for the train.

Here is an example of getting a 4-star weapon.

If you are playing with friends or have grouped up with people, you should try going to one of the loot locations because there will be plenty to share.

If your all alone you should try going for the vending machine strat because of the limited supply of weapons.

Also please comment if there are any other good loot locations.