WARNING - Try and complete the encounters as you are doing them, because it is a pain to do it at the end of the game! Specifically, it is vehicle delivery and out of gas encounters you are looking for. I had the best success in provinces where I had no other resupply points unlocked on the map. I fast traveled to a city, waited on a roof, and eventually they appeared. Had a few bug out, such as a buggy delivery stopping metres from the drop point due to 'innocent hurt'. You will have to wait around for a good few minutes, so be patient, listen to some good music or pat the cat.