The usual suspects for getting plastic have been already listed by previous posters.
The "other option" is ofc to try to get by without platic to build up the stockpile for the times you feel the need to use resources that take plastic. For a start, armor is currently not working on zombies, supposedly, so in 19.1 only advantage of using a steel arrow is, supposedly, the couple extra points of damage. Depending on the perks taken a Q5 iron crossbow with iron arrow (which uses feathers instead of plastic) and couple of mods has enough punch to down a "normal" (cheerleader, janitor, businessman, etc) radiated zombie sleeper with a headshot from stealth. And with shotguns - well - the normal buckshot (which takes paper instead of plastic to craft) works pretty well as well so it is always option to use these for clearing random houses and save slugs for special occasions.
A "hack" in this regard, if switching ammo type is too annoying - just have two of the same weapon on your toolbelt with different types of ammo loaded. You can paint them different colors, if it is otherwise hard to remember which is with which ammo. So that if you feel that you need slugs or steel bolts right now you can just switch to a weapon with one loaded instead of going through switching the ammo type.