This is the secondary objective to the fourth level. Like on the first mission you'll be runninjg out to get these and bring them back to the village, but to different places each time. There aren't as many to get this time though. Several are located at different points and a few are grouped together, but they are all in close proximity to one another. Just follow the black arrow while being as quick as you can and you shouldn't have a troubles.
*Has to completed before destroying the APC’s that attack the village.