How do I get more ethanol?

Outposts are the main sources of ethanol. However, you can get more if you manage to clear the outpost without getting detected. You’ll get 50% more of the outpost’s default ethanol reward. If you do get detected but managed to prevent enemies from calling in reinforcements, you’ll get at least 25% more ethanol. The amount you get will increase as you tackle higher difficulty outposts. Completing a Rank 3 outpost undetected will yield 450 per run or 375 if you're detected but no alarms were triggered. (300 default + 150 or 75 bonus ethanol, respetively) Farming ethanol will a lot easier if you already have a massively upgraded bow that can one-shot elites.

Other sources include tanker trucks. These are found along roads and will usually have escorts. You have to kill the truck’s driver first then deal with the escorts. Be careful when shooting at the driver since the truck itself is very volatile. If you damage it enough, it will explode. Once clear, you have to drive the truck to the nearest liberated outpost; otherwise, you’ll have no choice but to bring it back to Prosperity.

Another source is the enemy air drop. You’ll get a notification in the lower-left corner of your screen if there’s one near your location. These are easy to spot since you’ll hear the plane overhead and the crate parachuting down with a yellow flare. Usually, there’s at least a couple of highwaymen who will reach the crate first so take them out to loot the box.

Finally, watch out for scavengers. These highwaymen loot the dead so you’ll find them usually along the roads as well. If they see you, they’ll usually run away after deploying a smoke cloud but if you manage to jump on them, you can loot them for a good amount of resources including a chance to get some ethanol.