They are somewhat commonly found by checking the trash / recycling, desks, and lockers. It does have a few set locations however.
Unlike most items, it appears quite different in the map than it does in your inventory; see the image to the right.
As it is a "resource" item it has no quality, and as such all set locations are always salvageable.
|Sunset Mall||There are two in the construction zone on the east side of the map, to the south of the preschool.|
|Walker River||From the spawn, follow the river until you see the second wooden bridge; cross it to west side of the river, and it should be right there.|
|It's in the far west gas station, in the backroom on a shelf.|
|[Secret passage] Inside of the impound lot, along the north most side area.|
|Highway 99||From spawn, get onto the highway and follow it until you can't go any farther without getting off. The piece will be to the right of a red/orange car with a mattress on it.|