Plastic in Fallout 76 is hands down one of the most essential resources to farm in the game. It seems like Plastic is used for most things in Fallout 76, and in order to do things like bulk junk, craft certain chems or explosives, create ammo, weapons, armor, and power armor with their corresponding modifications, you ALWAYS need plastic.
There are some objects in the game that supply you with a bit of plastic, but you need a LOT of plastic in order to craft or bulk things in Fallout 76. Here are some of the objects you want to farm to break down into Plastic: