Plastic is pretty useful stuff, not only in making weapons, armour and building items, but in bulking raw materials to sell to vendors. If you’re not familiar with bulking go to a Tinkers Workbench and you’ll find ‘Bulk’ as one of the options under the Craft menu. There’s a long list of materials that can be bulked, mostly raw materials such as Acid, but also components such as Springs. For example, 30 units of Wood bulks down to a single unit of bulk Wood that can be sold (though not for very much) but each time you ‘bulk’ anything it uses up two units of plastic (you can also bulk plastic itself).
The good news about plastic is that it’s found in many junk items, however, very few of them have it in much quantity. Cafeterias are usually good places to pick up plastic as they often have an abundance of Cafeteria Trays, Plastic Plates, Plastic Knives, Plastic Forks, Plastic Spoons and Plastic Bowls. Also check out the adjoining kitchen for Bread Boxes, Cooking Oil, Salt Shakers, Pepper Mills and Coffee Pots.
The list below names all the junk items in the game that contain plastic. This list is taken from the official Game Guide, but be aware that it’s not always completely accurate.
Junk that contains Plastic (yield in bold)
Five: Bowling Ball.
Three: Bread Box; Life Preserver.
Two: Accordion; Banjo; Snare Drum; Anti Freeze Bottle; Antique Globe; Clean Globe; Baseball Base; Bone Cutter; Bowling Pin; League Bowling Pin; Cafeteria Tray; Cat Bowl; Dog Bowl; Plastic Bowl; Clean Dog Bowl; Cigarette Carton; Undamaged Cigarettes; Clean Pepper Mill; Pepper Mill; Clean Salt Shaker; Coffee Pot; Luxobrew Coffee Pot; One Ball; Three Ball; Four Ball; Five Ball; Six Ball; Seven Ball; Nine Ball; Ten Ball; Eleven Ball; Twelve Ball; Thirteen Ball; Fourteen Ball; Fifteen Ball; Feather Duster; Hairbrush; Home Plate; Industrial Cleaner; Plastic Gas Canister; Plastic Knife; Plastic Pumpkin; Suprathaw Antifreeze; Toy Alien; Umbrella; Clean Umbrella.
One: Autopsy Board Game; Rad Poker Board Game; Baby Rattle; Broken Doll; Doll Left Arm; Broom; Clean Broom; Clothing Iron; Cooking Oil; Coolant; Empty Coolant; Cue Ball; Two Ball; Eight Ball; Empty Blood Pack; Enamel Bucket; Energy Cell; Enhanced Targeting Card; Fancy Hairbrush; Food Dehydrator; Globe; Golf Ball; Harmonica; Mouth Harp; IV Bag; Jangles the Moon Monkey; Molded Plastic; Nuka-Cola Cup and Straw; Nuka-Cola Cup Pack; Pack of Cigarettes; Pen; Plastic Fork; Plastic Plate; Plastic Spoon; Preserved Cigarette Pack; Salt Shaker; Scissors; Shopping Basket; Shotgun Shell Casing; Toothbrush; Toothpaste; Untarnished Coffee Pot.
Remember that, aside from junk, you can also recover moulded plastic from some scrapped weapons and armour.
Whatever your source, be sure to have at least one rank of the Scrapper perk installed to help boost your yield.