Look at the sides of the houses at the Blueberry Acres area in Red County. The Quadbike will be at the side of a building. Alternately, go to the town nearest the northeast of the airport in the desert. The ATV is at backyard of the closest house towards the bridge in the southeast corner of the town. Go to the hilltop farm in the Red County area of Los Santos. There are two barns with lofts on them. On the barn to the right, if you jump at the correct moment, you can climb up and get a Quadbike. They respawn there every hour.