You can refill your ammo by visiting a store in one of your safehouses and then backing out (it’s slightly annoying that if it’s going to be so easy it isn’t just automatic), or by robbing corpses, and it’s fairly plentiful. Still, running out of ammo—especially for your often vital rocket launcher—in the middle of battle is a pain. It took me forever to finally buy the extra ammo packs available in the gear section of the store, but I should have made it an early priority. Definitely get it before worrying about RC car explosives—they just get stuck on rubble and are mostly useless.