C4 can be elusive at first, but can be traded for with the nomad traders. The nomad traders camp out in a couple of locations across the map. The closest is in a set of houses to the north-east. After you loot a couple locations and have a bunch of seeds make a trip out to one of the traders, I recommend grabbing a bunch of C4 in one trip.
You can use it on the various semis\containers on the map for supplies, on a couple of walls to access new ares, or on the special tech trucks to shorted the rescue timer. Chose your target and walk up to it and press (A) you'll stand way to close to the container, set off the c4 and get an achievement.