There are four golf carts available in the game. Three of them can be found in the lower right corner of the map around the golf course. The fourth one is in the upper left corner on an island which can be accessed through a bridge.
The golf cart, even once you obtain it, isn't accessible through the menu (jumping between vehicles). You can use it to move through various location when you don't want to drive the tractor. Its maximum speed is 26 km/h.
Golf cart #1
Once of the carts can be found in the middle of the golf course. It's quite hard to miss - rather distinguishable on the open space.
GPS coordinates: [1927, 1753].
Golf cart #2
The second cart is nearby the first one, on a parking by the golf course.
GPS coordinates: [1826, 1638].
Golf cart #3
The third one is a bit to the west of the previous one on the parking. It's at the back of a building with an ATM machine.
GPS coordinates: [1764, 1637].
Golf cart #4
The last golf cart is in the north, on an island. Access it through a bridge and you will find it behind a caravan.
GPS coordinates: [617, 149].