Rubber can take you by surprise (especially if you catch Grandma wearing it) most of the time you have little use for it in the game, but once you start crafting complex weapons and armour you might find yourself running short. This is especially true if you take on a base build that has a lot of wiring and electronics.
Unfortunately there are few junk items in the game that contain rubber. The most common rubber-bearing items you’re likely to come across are Extinguishers and Plungers. Most bathrooms have a plunger, and a decent-sized building might net you half-a-dozen extinguishers. The main trouble with the latter is their weight - collect too many and you’ll soon become overburdened.
The list below names all the junk items in the game that contain rubber. This list is taken from the official Game Guide, but be aware that it’s not always completely accurate. For example, the official list misses Coolant, which has two rubber, and TV Dinner Tray, which has one.
The highest yielding junk items are Kickballs and Basketballs. These aren’t common, but you can often come across large stashes of them in schools and amusement parks. Three good looting locations are the Grafton High School where you’ll find a container full of basketballs in a store-room off the gymnasium, and both the Tyler County Fairground (not the Public Workstation, the actual fairground to the north) and Camden Park have a decent amount of basketballs lying around, just find the Hoop Toss games at both locations and check any wheeled storage containers you come across.
Looking at the list below, three oddities are the Bonesaw, Saw and Spatula. Who would have guessed they have any rubber at all?
Remember that, aside from junk, you can also recover rubber from some scrapped weapons and armour. Moleminer equipment usually also scraps down to yield some rubber.
Whatever your source, be sure to have at least one rank of the Scrapper perk installed to help boost your yield.
Junk that contains rubber (yield in bold)
Three: All-Star Basketball; Basketball; Unfilled Kickball; Kickball; Deflated Kickball.
Two: Extinguisher; Plunger; Toy Alien; Coolant.
One: Bonesaw; Saw; Large Baby Bottle; Small Baby Bottle; Spatula; TV Dinner Tray.