There are many places that you find the parts for the bus, but what you will find there is random each time. While you will get all the parts you need by visiting each spot, the order that you get them will not always be the same.
The first location is in the Bus Station. Once you have created the Turbine, use it on the door immediately to the right of the work bench you used and inside the room here should be a piece for the bus against the right wall.
The second spot to find the bus pieces is at the Diner area and can located in the small shed to the far left of the Diner and of the gas station. To get in to the shed, much like the room at the Bus Station, you‘ll need to use the Turbine to open the doors.
The third spot is at the farm and can located in the small shed between the house on the right and the barn on the left. To get in to the shed, much like the room at the Bus Station, you‘ll need to use the Turbine to open the doors. Plop it down and then rush in to find the bus part sitting next to a tractor.
The final place that you can get a bus part is at the town. Once you arrive, go to the left of the Bank –across the street from the front door--and continue around the parked cars and the manhole cover surrounded by lava. The store to the very far left –that has a sign above it reading “Rare Books” will also require the turbine to enter, and will also have a bus part, should one of the previous spots not.
The ladder can be used to put on the rear, left side of the bus. Once you have the ladder, simply go to the hook at the back of the bus and you will be prompted to add the part there.
The hatch, unlike the other bus parts, can be used in two places. One of course being in the bus which to make things easier, jump towards the square in the ceiling of the bus, and quickly press X or Square to start building the hatch. The other place you can use the hatch is in the Diner. The first stop after the Bus Station, once you enter the Diner itself on the right side of the area, go to the far right corner and there will be a pile of rubble leading to a corner in the ceiling. Use the hatch there and you can then climb up to the roof where you can also purchase the brutal Galvaknuckles for $6000.
Just like a train, the Cow Catcher will be placed on the front of the bus once you have found it and added it. Probably the most useful bus item out of the three as it stops zombies from entering the bus from the front windows, the only windows where they just jump right in without having to break down any beams.
And for all your hard work, this is your reward. A pretty slick looking, kick-ass bus.