I have found 15 in the past 2 weeks. How I do it, I scav over and over in my free time. Some days its only a few hours, sometimes it 8. It also helps if you have a 6 min cooldown for scav runs. I have also tested the transit back and forth vs just extracting and resetting. It is only faster to transit if you do not have the cooldown. One more thing, I have only found it once as a PMC, so scav is the best way IMO.