High quality Buggy parts (Engine, Turbo, Suspension, Traction) and Breaks) can be obtained by using Blueprints and visiting merchants. Blueprints are unlocked on the Driver skill tree. This way, you can equip your vehicle in parts up to level 5. However, it is not the maximum, because there are also Experimental Military parts which can bring your Buggy to level 6.
To create level 6 parts, you have to obtain military Blueprints. The location of the Blueprints are shown below. Before you start searching for them, make sure that Crane has Military keycard. This item is obtained from Jasir at the end of The Gathering quest. Every Blueprint is stored in a military crate and you will need the keycard to open them. After obtaining a Blueprint, you just have to make sure that you have all the ingredients necessary to build the part.
Experimental Military Engine Part
The Blueprint can be found on a small island near the town in the eastern part of the map.
Experimental Military Turbo Part
The Blueprint is located in a wagon, in the northern part of the map. You will see it near the entrance to a lair of the Infected.
Experimental Military Suspension Part
The Blueprint can be found inside a wagon in the southern part of the map.
Experimental Military Traction Part
The Blueprint is located inside a trailer standing on the highway, in the northern part of the map.
Experimental Military Brakes Part
The Blueprint can be found inside a wagon in the south-eastern part of the map.