If you want a good boost of faction items, head to Panu City - Financial District. There are a lot in that area. Coordinates are X: 6076; Y: 13335
I normally buy the smaller helicopter in the Black Market and just pick up faction items one by one. Complete Strongholds to unlock more of the map to find faction items. I finished all 9 Strongholds before attempting this. They are little blue dots on your map, if it helps you can press LT while in the map and then select, "Hide All" then select, "Collects" to find them easier. You will have to do this each time you exit and re-enter the map. There are 3 types of Faction Items. Drug Drops, Skulls, and Black Boxes. Good luck it takes a while :)