They can be found in and used from the Signs and Items for Footpaths scenery window.
In RollerCoaster Tycoon
Litter bins may be placed on any path except queue lines. They allow guests to throw away their trash, effectively reducing the amount of litter in the park they're placed in. Litter bins can become full when used enough, requiring them to be emptied by handymen. They are most effective when placed around food stalls.
In RollerCoaster Tycoon 2
Litter bins work exactly as they do in the original game. In addition to the standard style, Litter bins come in different styles to match certain scenery sets, however these are simply cosmetic and function exactly as normal Litter bins do.