When it comes to vehicle mods in Fortnite, the cow catcher is easily one of the most fun in the game. The item can be outfitted onto the front of any drivable vehicles out in the field. As you might expect, a car with a cow catcher on it can be used to ram into opponents and props to deal heavier damage than normal. Aside from modding your vehicles, they can also be used as cover against enemy fire if you place them on the ground. It's a faster way to form barriers than building one.
If you want to find cow catchers out in the field, then be on the lookout for red toolboxes. These can be found anywhere in the open world and contain all sorts of ammo and vehicle mods. More likely than not, a cow catcher will be among your gets.
Red toolboxes can be found anywhere on the map, but your best bet to find one for sure would be in garages and gas stations in a given map. You can also find them in normal chests, albeit at a slightly lower rate. Either way, they're common enough.