Resource items come in four shapes; Armor Parts , Weapon Parts, Vehicle Parts and Money. All of them are boxes that you can pick up by running over them!
The resource boxes flash white and you should notice them from afar! In addition they have a tracker beacon and the small icon on the top left hand side of your minimap will show you the signal strength. Once the signal meter is full you will be shown the exact location by a white arrow.
Resource items are tied to location completion and you will have to pick them up in order to complete locations. But boxes can also be found outside locations. These have no effect on location completion percentage.
There are 2700 in total. 450 Cash boxes, 900 Vehicle Parts, 950 Weapon Parts and 400 Armor Parts!
There is no complete map as of today and I do not recommend using one. Finding these will come naturally as you progress through the game. keep to the instructions above if you should happen to have trouble finding items!